My son is also in the low weight percentile, he even dropped between his last two doc visits! She wouldn't even tell me the percentile, but it's below the 25th where he's always been. :\
He always has been a low weight percentile, since BIRTH. But it makes me nuts. I know I am not very rational when it comes to my son's weight and worrying about it. ;) Is he a super active little guy? Sometimes when I watch my son prancing from here to there ALL day long, I can't believe I'm shocked by his skinny tendencies! :)
Yes, being sick and the antibiotics can make him lose his appetite. If your doc isn't worried, then you should do your best not to worry. My son is 28 mo and I have slowly learned that he has appetite "cycles". He'll eat well for a few weeks and then not so much for a few weeks. When he's in a low cycle, yes, I feel like he'll wither away and die and when he's eating well, I'm delighted beyond belief!
Def don't overreact. I can tell you from first hand experience that while your son may cooperate now, as he gets older and exerts more of his independence when you push him to eat he will pull away and refuse even more. :) We have just been working on this for the last 3 weeks. I was trying to get him to eat ALL the time and he was finally refusing to come to mealtime at all. Now that I act like I don't care (I SOOO care) and even limit his snacking (kills me to limit his food!) he is properly hungry and is happy to eat!
This video REALLY helped me to chillax, I have literally watched it about 10x. It really comforts me when he's in a low eating cycle:
http://www.youtube.com/watch?v=eQ3sC_4ga1M
Good luck mama! You're doing a great job and some kids are just thin and active. Think about your son's intake over a whole week even a month--not meal to meal. Kids eat when they're hungry! They won't starve. Even we skip meals sometimes. :)