Losing weight gained during Starvation Mode? Different? Please help!

I know that things take time, but this time things are working differently than they did even just a year ago, and it's bothering me. Here's my hypothesis:

All my life, all I had to do was up my exercise to lose weight. My eating habits are overall good, though not perfect. If things were going to pot, I'd buckle down and exercise more and get things under control.

Last March, I was at one of those points again and decided to upp my exercise. I love my gym and the classes offered, so  I went M-F, for a total of 5 hours/week of vigorous exercise. Afte a couple of weeks, the weight really started to fall off, as expected. I was following WW Points, so come April the weight stopped falling and I started gaining, due to Starvation Mode. I didn't know about this effect of lots of exercise, so it was 3 months of feeling like crap and gaining weight (still going 5 days a week!), before I learned what was happening.

Did well over the summer, getting enough protein and calories to not be in Starvation Mode and lose weight. Then, come November and my schedule went south. Kept up with the exercise, but I was unable to keep my calories up. I figure in a 3 week period I gained 10 pounds, which I'm now trying to lose....again.

 Clothes that have fit me for 3 years, through other ups and downs, now don't fit, including tops, my bras and sweats. I don't look that much bigger, and my measurements aren't that far from previous.

Finally, here's my question: Is it harder to lose weight gained during a Starvation Mode due to the effects on your overall Metabolism? Why am I now gaining more weight than I ever have in the past, when I'm working out diligently, and my calorie intake is 1600 ish for exercise days, 1200-1400 on non-exercise days.

Part of me feels like all this effort is useless, the other part knows I need to keep doing what I"m doing. But, is there any reason why this time is so different? It's really been bothering me, because I wonder if there's something else I should be doing that I don't know about. Because exercise and diet ain't working as they should! (PS: Thyroid is under control, and I'll be 40 in May.)

Thanks in advance!