Congrats on being a new grandma! The biggest concern is the baby gaining weight and growing. Both a cold and the amoxicillan can decrease appetite, and the amoxicillan should be given AFTER feeding, not before. I am surprised they gave an antibiotic at such a young age, most peds now will not unless the child has a life threatening illness, as the baby is still relying on antibodies passed on from mommy.
Here is what I have learned after four children ages 4-26, and five grandchildren:
They will eat according to their own needs, as much and as often as they need, as long as they are gaining weight and growing, no worries. If they aren't gaining weight and growing, then a doctors advice is needed. With my first child we had to set an alarm and try to feed her every two hours around the clock. At around one month she fell asleep for her nap and I couldn't wake her for over four hours. Off to the ER we went where they couldn't wake her for another two hours. Turned out she was exhausted, imagine that! LOL She didn't take two ounces at a feeding until she was five months old. My next child was taking 3-4 oz every two hours from the first day. The doctors told us not to feed so much, I ignored them. Both girls grew up healthy and fine. All of the other children have fallen in between.
We did discover that a change of formula for the picky eaters helped. My youngest daughter and three of my grandchildren refused any formula with ARA/DHA and would starve themselves. They all preferred Nestle Good Start. Good luck.