infection or normal period?

I'm 6 weeks pp and my husband and I had sex. I started the pill at 3 weeks pp so my husband didn't use a condom. I'd stop bleeding after the birth around 2-3 weeks pp but the other day when we had sex I began spotting bright red. I thought it was a coincidence and my period had started. now full flow but it smells fishy so now I'm concerned it's an infection? I've still got another week left in the pill pack and I've been bleeding now for 6 days and have pcos with erratic unpredictable bleeding anyway, which is usually controlled with the pill. I live in the UK where my health providers say we can resume sex whenever we feel ready.