It's definitely not too early - do what's right for your family, and don't worry about what other people think.
I had to stop nursing my daughter at 12 weeks because I had to start chemo. OK, better stated, I chose to start chemo to save my life and be her mother. Her having a mom was more important to me than her having Mommy's milk. I had exclusively nursed my son to 12 months. Our daughter's been in the 90%+ since about week 8. We initially mixed frozen breastmilk with formula and then went cold turkey on formula for her. She did great.
Formula won't cause any stomach issues that aren't already there. It may be an adjustment period until you find the right formula (ask your pediatrician for samples of different brands if you can) and see how your baby reacts.
Good luck!