I am not sure but I heard that after 30, some hormonal changes would cause women to gain a bit of weight. It happens to women after 30. I am experiencing that at the moment. I'm only 33 and I've gained 12 pounds without realizing .I guess, it's just more reason to take care of ourselves so that we don't gain more.
I'm still not giving up, but I have also learnt to love my body as it comes. We can't expect to have our 20 year old bodies. It's not the size that matters it's what you can do with it. LOL! As long as we are healthy and able to do physical activities without running out of breath, we should be ok.
Wishing you all the best in your quest.
First off, at 5'5" and 136 lbs, you aren't overweight; I've put in a bit of a height/weight chart that would apply to you.
Height Small Frame Medium Frame Large Frame
5'5" 117-130 127-141 137-155
That said - yes, hormonal changes can cause weight gain, but I don't believe it's true that when a woman hits 30, she automatically gains weight. I am 60 and I didn't gain much weight to speak of until I was in my late 50's and my thyroid completely stopped working. I did bounce around weight wise for some years, but that's because my thyroid was causing me to go from hyperthyroidism to hypothyroidism.
I would suggest a couple of things......1) if you are gaining weight rapidly, you should talk to your doctor and have him/her do some thyroid tests to make sure you aren't hypothyroid. In addition to that, there are also other medical issues that can cause weight gain/inability to lose, such as PCOS, insulin resistance, etc; so you might want to get checked for those as well.
2) It would be a good idea to keep a food diary for a while to make sure you aren't inadvertently changing your eating habits. By keeping a food diary, you can record every morsel that goes into your mouth (including liquids) to see how many calories you are consuming in a day's time. Quite often, we end up consuming more calories than we actually think we are.
You should also make sure that you mix up your exercise and calorie intake. By doing the same exercises day after day, you body becomes used to them and they are no longer a challenge. Same goes for your calories -- increase your intake by a couple hundred for a day or two, then decrease by the same amount for a day or two, so your body won't know what to expect.
Good luck to both of you.