Yo! I agree with the replies you've received, Defcon! Please continue to check the Harris Benedict calculator (I put it into a spreadsheet on my hard drive to use whenever my weight changes) and get either a good calorie counter in print or a website and/or talk to your doctor or a nutritionist. I lost 100+ pounds on 1800 a day ... although I've had a slight regain, I know that I will never want to go below 1500 a day. Everyone is different though and you need to get your eating right for YOU ... but considering your menu makes me worry ... (sorry, you did ask for input)
Just off the top of my head, look at the calories in your menu:
Rolled Oats (150 for a 1/2 cup serving), Glass of Water (0), Apple (100).
Lunch - Some Salad, medium size bowl, with 'light' dressing (100-200), Glass of water (0).
Dinner - Some Steamed Veggies (maybe 100?), and a skinless boneless roasted chicken breast (250), Glass of water (0).
Heavens! At best that's approximately 800 calories; if this is a typical day, you are eating starvation rations and it's no wonder your metabolism has slowed down. You cannot possibly be getting the nutrients your body needs to be healthy and strong.
Please, eat more than a muffin ... and are you getting calcium and some healthy fat? It is old science that fat is bad for us ... your menu has very close to zero fat and you really need more of the good kind ... there are also essential fatty acids your body doesn't manufacture that you need daily ... your menu is a good skeleton diet but you need to flesh it out and you'll start to lose again, IMO (since you asked)!
Eat!