I am having the exact same problem. I am soooooo frustrated. I know it isn't menopause in my case either because I am only 33. I notice that my body does this every so many months...like 8. I think what happens is your body gets used to doing the same old stuff and stops responding. My cousin...who is an aerobics instructor...says you have to shock the body every once in a while. I was doing 4 days of cardio and two days of weights....light. I am still doing 4 days of cardio, but one of them is Body Max, so with this workout, I have added an extract weight workout as well, even though the weights are a bit lighter. On my other two lifting days, I have made my weights higher. I was always worried about getting bulky, so I have always used light weights. I am getting over that fear after reading this forum. I have also noticed something with my body that doesn't make alot of sense, but, when I do alot of cardio, I tend to gain weight. And it's not muscle either because my stomach and waist line expand considerable. I have no idea why this is. Maybe I am eating more than I realize. I was running more than an hour 3x a week and gainning weight. I was so frustrated. You'd think I would be a stick. So I cut it back to 45 minutes and I've started dropping pounds again.
I can say this, I have always been told that when you hit 30, things change with your metabolism and body. I never believed it, but boy is it ever true. I swear every year on my birthday, my metabolism takes a nose dive. I have to work so much harder now that I ever have and I have to watch everything that goes in my mouth. I look at it this way, it is going to make me a healthier older adult.
Anyway, I guess what I am saying is maybe your body needs a change of senory. Try to shake things up a bit. I don't know how much weight you are doing, but maybe go heavier. I have always had trouble with my middle...I tend to be thick there....until I started doing some stability ball work for my core. I am still thick, but not nearly as much as before. How often do you do stomach. I notice that if I do stomach too much, I get thicker. I only do it about 3-4x a week.
Okay, now that I have totally talked your ear off, I am shutting up now. I hope I helped you a bit.
Kim