Why are my nipples hurting?

What are some reasons for my nipples to hurt? Do you think I could be pregnant even though I have been on combination birth control and use condoms everytime I have sex with my boyfriend? I have not had any other symptoms other than my nipples hurting. My next period is due July 19th and I got a period on June 19th which was dark brown blood plus some clotting. I have not had sex since the June 5th. Any advice would be helpful!