J Butler, there are several things to check out here.
Sudden bloating or increase in the abdomen can be a symptom of ovarian cancer so really should be checked out and mean a visit to the GP.
If it is gradual weight gain which has happened over years, then it's likely due to your loss of estrogen. It is well known that low estrogen slows metabolism down and causes fat to be deposited on the stomach and not on the bum/hips (or less there). That is the shape of a menopausal woman not on HRT - a big meno belly, often still with slim legs and bum.
Before you get told it's too late for you to start, you really should consider beginning HRT.
Lastly, you should also ask your GP to check your thyroid because hypothyroidism is very common in women as they get older and also causes weight gain. Your GP will just test your TSH though, and you need T3, T4 and thyroid antibodies tested - so you really need to test your thyroid yourself via Medichecks . com if you can. The Advanced Thyroid Panel with them can be done at home with a finger prick test and posted back.