If you are pregnant the light pink color could have been slight bleeding due to your cervix getting irritated from sex. This is not uncommon during early pregnancy.
I'd say that you need to do a test, a lot of your symptoms do sound like early pregnancy symptoms, do you usually get sore breasts before your period? if not then this is a big pregnancy sign, a lot of women notice that.
The pink fluid could be implantation blood, yes.
If you do a test and it is negative, I would recommend you go to a sexual health clinic to get checked over, any pain and bleeding from the vagina after sex needs investigating, especially if its happening a lot.
Good luck