I used a sex toy to mastrubate with. could my semen go backwards? it was a tight fit for my hard penis the first time, so i don't know if i am infertile.

Probably fertile: The sperm could have gone backwards - this is called 'retrograde ejaculation' and the sperm goes backwards into your bladder and comes out later when you pee. It is unlikely that having this happen once would damage anything or cause infertility later on - assume you're fertile if you're not trying to have a kid. Obviously if you have abnormal swelling, bruising or bleeding then get checked out.

Check Semen analysis: If concerned about fertility, best to obtain semen analysis to check semen volume, sperm count and motility.

Doubtful: The sex toy doesn't appear to have caused any permanent harm. If there's any concern then have a semen analysis checked , but what you described doesn't raise any concern.