TMI: Postpartum sex?

Hi, so my baby is 4 months now. He was born via a planned C-section (due to his being a footling breech), my incision site was tender but getting better slowly, I am breastfeeding him.

My question is about postpartum sex as this is my first baby and I don't know whether what I'm experiencing is normal or not (TMI and NSFW warning).

Since having the baby my husband and I have tried to have sex maybe 3 or 4 times and each time it's extremely painful for me because eventhough we are using lube, my vagina is so tight, to the point where I can only just about take the head of his penis inside me no matter what position we try. At first I thought it might just take a few more weeks/ months, but I'm starting to get a bit frustrated now, it hurts so much that I feel like I've been re-virginised.

I have Googled it but coming up blank. I don't know whether this could be because of the C-section or breastfeeding and whether this is normal or I should see a doctor?

Thanks in advance :)