5'5" almost 40 trouble maintaining

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So, I joined MFP about 1.5 yrs ago to help me lost 15 lb. I've never been overweight, just at the higher end of normal, and I felt unhealthy. My highest was 144 lb. I was successful using MFP and my weight got down to a low of 124lb. I stayed there for a while until last summer when I seemed to settle around 128-129. I was ok with this weight. I was and am fairly active. I run, do weight training videos, and work as a nurse. Over the winter, without changing things, I've gained up to 136 lb. I can't seem to maintain. I lose or I gain. Help!!