You have a very nice observation which you have mentioned in your history.
The reason for your
pain during intercourse, in fact it seems to be at the insertion, which seems to be due to the bumps you have mentioned at the
vulva appearing with the start of menses and then disappearing.
These bumps seem to be the infected Bartholin's glands. These Bartholin's glands are situated in the vulva just inside the lips near the vaginal outlet. When they get infected, they swell causing pain. Usually they form
abscess. This infection increases during menses due to the presence of blood which is a good culture for organisms.
When the abscesses are formed, they need to be incised and the edges need to be everted (marsupialization).
These Bartholis abscesses or infections are very painful and seem to be the most like cause of your painful insertion during intercourse.
Please get a Gynaecologist's opinion. If needed, get the surgery done. It is a minor surgery. Continuation of antibiotics post surgery is the key to get completely cured.
Also, please ask your partner to get checked from a venerologist for the probable source of infection he may be harbouring. The most common source of this infection is sexual route. If he is also infected, get him also treated with proper antibiotics SIMULTANEOUSELY.