After me and my girlfriend had sex, she started her period, normal in color, back pain, she broke out, just like normal but few hours later she stoped?

Menstruation: can be very varied, especially in a young woman still growing in hormonal terms. Spotting and irregular menses can be a normal part of that, or it could mean there are other factors involved such as infection, stress, lack of proper nutrition or sleep, seasonal temperature, or other hormones (thyroid being a prominent example) playing a part. If the irregularity continues, see a GYN or family Dr