What Causes Vaginal Burning Sensations After Ejaculation?

hi i have been on cerazette on and off for 4 years now. I have had two partners and with each partner I get a burning sensation in my vagina after them ejaculating. I am very worried about this and I am not sure what is wrong. When I first started taking the pill it was fine barely any burning also when I use lube it does not burn very much but without them it burns like hell! please help