Ouch!

Lately when myself and my long term partner have sex it's begun to really hurt me and I wouldn't have a clue why since 1. We've been together for a long time now and 2. It's only started happening within the first month. 
I'm just curious as to whether any other ladies experience sudden discomfort during sex suddenly and unexplained and if it was something that I should see a doctor about or if it's just natural?