How after 3 weeks of STRICTLY sticking to a low calorie budget and running 2.6 km a day 5 times a week my scale says I'm 6 POUNDS HEAVIER and my measurements haven't changed a bit and my clothes feel tighter.

This is getting even more depressing than before I was trying. At least when I ate anything and didn't move I was at least maintaining my weight/size!

Can someone explain to me how this is possible and please don't say 'underestimating my calories' because I count every bite every lick and I haven't had sugar in 2 months.

I'm lost here. I'm really tired of being this heavy, but I'd rather at least STAY at 200 pounds and not get bigger as my efforts increase.
I feel like I'm going insane.

What type of foods are you eating? could it be muscle weight? x

I agree. With me, I dont lose weight if I am eating processed food...no matter how low the calories are. When you say you are on a low-calorie budget, what is "low"?

Where are you in your monthly cycle? That coupld sway you 5-6 pounds easily. And as far as measurements go, I'm a big girl, so I measure once a month and even when I'm losing 10-15 a month, I will only see .5 to 1" loss in inches. They move slowly for me and my large bone structure.

Don't get discouraged!
Are you drinking a ton of water? You could be holding water because of exercising and dehydration. It can change things a LOT. Try to drink as much water as you can in a day--like 4L and after a few days try weighing yourself again?

Are you always weighing on the same scale , at the same time, same conditions ?

Maybe post a sample day of food, also? Make sure weigh-ins are same time of day, same clothing(or lack thereof).

Agree with 170starting How low is your calorie intake? What is your RDI? If there is too great a difference between the 2 you won't see much progress.

To answer all your questions:
I am 'hoping' it is muscle weight. I eat 'clean', usually I'll have a half cup of All-Bran or oatmeal with skim milk for breakfast, I work out in the morning and around 11AM after my workout I'll have a smoothie made with a cup of frozen fruit, a half cup of greek yogurt and ice. I eat lunch around 2:30PM, I usually have a bean salad with tuna and a few different raw veggies chopped into it, and I'll put a few tablespoons of quinoa or rice in it as well (sometimes I pre-cook and freeze a batch of rice or rye beads so I have good whole grains easily on hand - I'm not sure how the freezing affects the nutritional value of it though).
Dinner is usually around 7PM, and is either a portion of mean with a green salad / fruit+veggie salad, or a stir-fry of sorts; vegetables and meats in whatever mix possible.
I have a 1.5L jug of water that I keep in my fridge, I refill it after my workout, and in the evening (and that lasts until the morning, and so on). So I do drink a lot of water, I also eliminate it so I'm not thinking it's water weight either.
When I say low-calorie my goal is to consume 1200 calories a day but my max is 1500 considering the type of foods I eat. Does anyone think it may be too much?
I usually notice I gain a few pounds during my time of the month and go down after, but that would not be the problem for this week. I also have a medium/delicate bone structure according to my physician. I know it could go differently every time but when I lost weight before I saw a noticeable change in my measurements, which is not happening right now.

Also, yes, I always weight myself without clothes on, after I go to the bathroom, on Wednesday morning. I use the same scale and put in on the same tile in my bathroom in the same position/direction.
:S

This happened to me when I dieted using prepackaged meals. It turns out that I was getting too much sodium and not enough water. I was retaining water and bloated. I didnt realize I had to drink more water in order to stop retaining water.

According to Fatsecret, moderately active with your stats to lose 1 pound per week would require 2100 calories. 1200 is too low, but 2100 is probably too high. Try going for a few days at 1500-1600 and see if you make progress. You also may not be getting enough fats. You should be getting about 225 calories from healthy fats.

Since reading your other comments it may be muscle. Also could you be eating not enough calories and your body is in starvation mode?

i agree with starvation and 1600 calories. maybe try adding some almonds? 10-15 a day? healthy fats are not your enemy!

Muscle is denser than fat. While you may be gaining some muscle mass, this usually means the clothes are looser, etc. Since you mentioned that your clothes are tighter, that indicates that you are not just simply swapping fat for muscle. Besides, you're not going to gain 6 pounds worth of muscle in a week. That kind of muscle growth takes months.