why is this?

Hi guest,

It could an allergy to the condoms or any lube you might be using.  It could also indicate an STD.

See your doctor.

Yea, allergy is possible. But also, i suggest you see your doctor to rule out a yeast infection. 

Women with yeast infection may develop symptoms including:

Pain/itch/burning at vulva area

Pain/itch/burning in vagina

Pain during intercourse and/or urination

Cottage cheese like vaginal discharge

Swelling and pain around the vulva

Yeast infection is not something serious in most cases, but lots of women are bothered by recurrent infection and can't find their way out. If you do have one, you'll get prescriptions.  There's a herbal medicine named SPAM also works well, you can google it and have a look. Hope this helps!