Hi, i've just started back at work and am still...

Hi, i've just started back at work and am still breastfeeding. I need to express everyday as i get very full. Today however, when i expressed i had blood in the milk. I do not know if this is because my daughter is teething and may have bitten me when she feed yesterday or because of the expressing. Can anyone tell me if this could be mastitis as i have never had it and my daughter is close to 11 months. I do feel pains in my breasts at times also.