Cycle and swelling

To me of 26 years, a menses since 15 years, pregnancies were not. Height 156, weight 43.
The whole life cycle was extremely unstable, fromsuper short (15 days) to super long (45 days). The menstruation itself also differed in duration (from 4 to 10 days), as a rule, menstruation is plentiful, often painful. Sometimes (not every cycle) swells around the chest before menstruation for a week and a half, increases in size, volume, hurts. About 4 years ago, all the same decided to find out what's wrong. Analyzes for hormones and infections are normal, uzi is normal, concurrent examination with an endocrinologist (also uzi thyroid, tests for hormones), nothing but endemic goiter of the 1st degree is revealed. And I was told that this means that this is my individual cycle, and nothing you can do with it, just take OK at will. There is no desire, because when I receive OK, I have a large weight gain.
Now about the problem itself. In the last two cycles, the duration has increased (relative to the average). 40 and 36 days. In this case, two weeks before the onset of menstruation, the abdomen from the stomach area to the abdomen is significantly increased. The intestine is normal. Just a feeling that inside the water, which presses from the inside of the stomach, from which it hurts. For two weeks I go bloated like a frog. After the onset of menstruation, the stomach returns to normal. In general, the sensations are the same as with swelling of the chest. But the quality of life suffers much more. At the native sister such problems with a stomach or belly occur or happen at each cycle some years. Sister is younger than me.
Advise, please, what analyzes it is necessary to hand over, what actions to undertake (uzi, any other inspections), and what the possible or probable reason. It's impossible to live.

You are deficient in body weight, this can be the cause of an unstable cycle.

Well, if there is a reason (you did not get the tests and ultrasound), but the goal?
There are a number of diseases, when things do not, the cycle will never recover. Also it is necessary to accept COC.

Treatment - most likely hormonal contraceptives will be prescribed, allowing you to have a 28 day cycle, regular menstruation.

Thank you Doctor!
Yesterday was at the gynecologist and has made US. Nothing critical was found on the armchair, everything is normal (the microflora is OK, there is no erosion), the ultrasound is also good, without pathology, the description was left in the clinic, since there was nothing like that, but remembered that the endometrium was 6 mm on 11 day of the cycle.
In the next cycle I donate blood to hormones (LH, FSH,estradiol, prolactin, TTG, T4cv, testosterone, DEAC). It's not earlier than a month. most likely it will work if the cycle remains the same. The results will be laid out here and an ultrasound.
Well, the gynecologist still offered COC (Jess).
The purpose of my research was to find the cause,on which I have before my menstrual bleeding so belly, and eliminate this cause. Because it is very difficult to live. By itself, the irregularity of the cycle does not bother me and does not bother me. I'm already used to it. But with a stomach or belly so was not earlier.