Congratulations on your weight loss so far! You are so close to ONEderland!
As to the depo, I would schedule an appointment with your OB/GYN right away!
Any kind of spotting should be noted to your doctor immediately...it might be nothing, but it's not worth the risk of leaving it alone.
I know your pain...I'm plateauing as well, currently. And I, too, lost a lot of weight quickly at first.
Have you measured yourself? Do you have any access to your fat percentage? Measuring yourself may help you have a better idea of what is going on with your body. You will know for sure that you are building muscle and losing fat if you can get your fat percentage, either with calipers or a more finely tuned instrument.
Try changing your amount of starch and fruits each day to see if that jolts your weight loss forward. Do it one step at a time...i.e. add one starch, or take one starch away, and do just that for one week, and see what happens. If it's not working, try something new.
Make sure you are drinking enough water and eating enough protein.
Gaining muscle is a good thing, so if that's what is happening, it's time to rejoice!
If you poke around on this site, you'll find a pic of five pounds of muscle vs. five pounds of fat and find that you will look amazing with as much weight in muscle as you now have in fat.
Failing all else, some people on this site have found that doing a week of Phase 1 can help jump start their loss again. With the depo and the workouts, if you aren't drinking enough water (64 oz a day, at least!), you cold be retaining water, and the Phase 1 will get it back out of you.
I wish you lots of luck! Please let us know how it goes!