Finally pregnant!! 6 weeks and 1 day!

We went for an ultrasound with our fertility doctor and we measured 5 weeks and 6 days.... we heard the heart beat!!! Omg it was amazing... but the docotr said the beat was a 89 she said it was a little lower than they want... what should it be?? How low is it? I know my doctors job is supposed to find things that are wrong but I feel like I'm just finding out great news and then they throw crappy news at me and it makes me go nuts.

Also, would I know if I was having an eptopic pregnancy? She told me to call or go in if I have pains... does she mean I would feel bad pains or just light cramping? Can you tell I over think everything? Someone help, please!