I'm struggling to have sex because of a strange, overwhelming feeling in my lower back.

TL;DR - PiV sex, especially from behind, causes an overwhelming (but not painful) feeling at the back of my pelvis. It sometimes makes my core muscles tense rapidly. Any ideas what it could be??

When my partner and I have penis in vagina sex, I get a weird, horrible feeling in my lower back. It is so intense that we sometimes have to stop. I'd like to learn what it could be, so we can look for a solution or ways to manage it, because sex is great and I want to enjoy it more!

Sensation: the feeling is hard to describe - the most accurate I can be is 'overwhelm'. It's like a building of sensation that gets worse whe he thrusts in quick succession. It isn't pain, but it is a sort of awful intense ache. There really isn't any other feeling I can compare it to.

Location: it's centred between my PSIS/Dimples of Venus. At its worst, it travels up my belly/lumbar area and causes my core muscles to tense suddenly - like the physical clenching experienced when vomiting (but I don't feel any nausea).

Trigger: I feel it regardless of position, but the worst are standing sex from behind with me bent over and doggy style. The feeling builds with each thrust, so 5+ thrusts in quick succession can be unbearable at times.

Theories: looking online just gave me info on back pain, which this definitely isn't. I have experienced sex-related trauma, but this doesn't feel related, I haven't been physically injured by sex and there's nothing 'special' about the aggravating positions. My partner has a very large penis (length and girth,) but I don't feel like my vagina is being excessively stretched (??) or anything. He has, on occasion, hit my cervix which is very painful, but a completely different feeling and location.

In case it's relevant: I'm in my mid 20s, average weight, taking an SSRI, progestogen pill and we use a condom. Sex varies between approx 1/month - 1/week.