Help ! Breastfeeding and I might need to stop

I got a tooth infection a week ago now , the dct prescribed antibiotics amoxicillin. I have two more days left to take them but I noticed my baby has red blotchy / patches skin around his neck and torso area. Is he getting a reaction to the antibiotics? I’m EBF so now I’m worried I started to give him some of my freezer stash but again I did not have much to start off with. I’m running low and was wondering can I do one feed with breast milk from the bottle and then the other feeding with formula? Will this mess anything up ?