TTC While Breastfeeding

Kayman
My son is 3 months old as of June 9th. I am wanting to get pregnant again, but I am not sure if I can because breastfeeding prevents it as well as birth control. I had a period about 6 weeks ago and none since, but my pregnancy tests keep saying negative, so I imagine its because I'm back to breastfeeding entirely (I mixed in the bottle for a little while before I got my period) and possibly because of stress- I've been very stressed for the last two or so months. Is there anyone out there who knows a way to get pregnant without taking away breastfeeding or adding in the bottle? Also, my son won't take pacifiers, so that isn't an option either.