What Could Cause Severe Pain During Periods?

Heavier pain than usual at the start of period (First period after starting active sexual life) Hi, we are a newly wed couple, we had sex for the first time a month ago (first time ever for both) and have had no problems at all until today when she started her period (right on time as expected, she is very regular) but she had a lot more pain at the start and her bleeding was a slightly different color and consistency as usual, after some hours and taking 2 Advil pills for the pain, it seems everything returned to her normal symptoms. What could this mean? Pregnancy? Infection? Take in mind we only use natural contraception methods (no pills, no condoms, no injections): Temperature based registry and use of a fertility computer (Baby-Comp), we have been very careful to only have sex on her infertile days.