I have no clue when it's time to go to the hospital.

I know that contractions feel like menstrual cramps and you feel pain in your back. which I'm having both , but idk if my back pain is from this sciatica pain or what . (and I'm guessing I have sciatica, but I'm pretty sure I do , cause I have all the symptoms.) but baby is really active so that's why I'm not too worried but I do have a lot of pressure. and it's hurting to walk . is it worth going to the hospital , or is this normal pregnancy stuff ? I'll be 36 weeks on Thursday and this my first .