Ditto this - don’t self-medicate a foal. Use medications only on advice of a TRAINED LICENSED VET.
Just like in pediatrics in humans, kids react WAY different to medications than adults do.
Same with foals. They’re bodies are not immunologically developed like an adult. The wrong kind, dose, and length of treatment of antibiotics on a newborn foal can have powerful consequences.
Antibiotics are not specific. They wipe out ALL bacteria sensitive to that kind of antibiotic - both good and bad. Wiping out the good bacteria interfere’s with the foal’s ability to digest the milk from nursing.
Have a vet assess the foal who is experienced with treating foals. An ultrasound, as others suggested, is very helpful to seeing if an infection is brewing, let alone an abscess, which can sometimes happen.
Sometimes, I have had a vet prescribe antibiotics fed to the MOM and thus the foal gets some of the antibiotics through her milk and that has been sufficient for the foal. Other times, the vet will choose to treat the foal directly with an IV dose. Depends what the problem is.
You can tell a problem is brewing long before any fever starts is if you touch the stump and area around the stump (have you hand wet with disinfectant before you touch for cleanliness, and then disinfect stump afterwards) and the foal reacts in pain. Not like a little irritated foot swish that says, “oh you’re tickling me”, but a real swat with the foot or a jump sideways with eyes widened and head up that says, “OUCH! that hurt”. Always better safe than sorry.
Prevention and early treatment = better chances of great results.
No treatment/late treatment/improper treatment = iffy prognosis.
$100 spent for the vet is better and smarter. Good luck with your cutie pie! 
