Hi cactuslover – so sorry to hear you’re having this experience! I’m glad you don’t notice this happening during sex, but having lingering pain for days afterward SUCKS.
I think your first step should be to schedule a gynecologist appointment if you haven’t already done so. A gyno is your most likely resource in diagnosing any issues with your vagina’s size, tissue, etc. and recommending the right solution. We’ve had other people on the forums mention this same issue and get prescribed things like estrogen creams to strengthen their vaginal walls or other people being recommended to dilate, so that is your best path for figuring out more specifically where to start with the problem-solving.
Let us know how it goes – I hope this problem is short-lived for you!