WHAT is wrong with me?!?!?

Before I was pregnant I was 135lbs. Then After having my son 3-years-ago I was at a steady 145lbs. Well, the last 4 or 5 mos or so I gained weight. There was no change in my lifestyle and if anything I eat healthier. I'm not a junk food junkie, but I do drink coffee every day. They are ALWAYS skinny mochas though (non fat milk, sugar free syrup) with no whip or extras. I have also been doing stability ball training lately and doing crunches. You'd THINK it would cause weight loss, but noooooo. This morning I stepped on the scale and it said 154lbs. WTF? About a year and a half ago my doctor put me on a small dose of levothyroxine for underactive thyroid. It hasn't been checked since. Do you think that is what could be causing the problem? Advice please??? I don't want this to get out of hand.