Advice for hitting a plateau?

When I first started watching my intake with MFP and working out regularly, I was having great success. I wasn't losing a ton of weight, but I don't have a ton to lose... so my rate of 1/2 pound to 1 pound a week was perfect! I went down a pant size, then it just stopped. I feel like nothing I am trying is getting me off of this plateau! (I am not losing inches either, by the way) I even started shelling out money for a personal trainer, and while I am seeing a major improvement in my fitness levels, my body doesn't seem to want to show it. In fact, the past couple of weeks I'm actually GAINING some weight. I am going to see my doctor to get a check up and make sure it isn't something like my thyroid (my energy levels have been really low too), but I wanted to see if any of you had experienced anything like this. Any advice would be appreciated!

This is a little bit more about my diet/exercise plan:
- I work out 6 days a week and am training for a half marathon. A typical week looks like this: Long runs of up to 11 miles on Sundays and 3-5 mile runs sometimes interval runs on Tuesdays & Thursdays (I have bad knees, so any more running days during the week gets them acting up). On Wednesdays & Fridays, I do 30 minute sessions with a personal trainer doing heavy lifting in a kind of circuit training style, and I'll usually do some kind of light cardio before or after. On Mondays, I usually do the elliptical & weights (heavy but not as intense as my training session) at the gym or I'll do a video like 30 DS or TurboJam. If I'm feeling particularly beat up from a long run, I'll just do light yoga and a walk on Monday. Saturdays are my rest day but I usually still try to walk, do light yoga, or do something else to move a little bit and stretch my body.
- I have MFP set to a sedentary lifestyle because my job requires me to sit at a computer pretty much all day. While I workout a lot, I spend about 8 or more hours a day sitting I have it set to lose 1/2 pound a week, so my goal is to net 1270.
- I eat back my exercise calories.
- I try to eat small meals and eat 5 times a day.
- I try to eat as clean as possible, keep a close eye on sugar, and do very little dairy.
- I usually have one day a week where I go over and let myself indulge a little bit.
- I weight about 146 (I had gotten down to 143) and am 5'6. I'd like to get to the 130s, but really if I can start losing some inches, I'll be more happy about that than the scale number.

I'm afraid to increase my calories because I spend so much of my day sitting. At the same time, when I reduce my calories by not eating back my exercise calories, I feel sick and light headed. Any ideas / any one else had this problem?