Sex help needed !

Naomi
The last few couple of times I had sex with my boyfriend my vagina really stung for a good half an hour afterwards and I'm not sure why ? 
We hadn't seen each for a while so not sure if I just need to get used to it or it's something more serious 
Any help/information is much appreciated 
Also just started on the three month contraceptive injection a few weeks ago if that makes any difference