It looks like you have been given Metacam for dogs, there is also a cat variety they are just different strengths of the same active ingredient. sometimes referred to as Loxicom.
its a painkiller & anti inflammatory. this is a normal medication & i have used it many times over the years for my girls & had no bad side effects (except the guinea trying to steal the syringe as they love it so much!)
it's easier to dose with a 1ml syringe, but the metacam for dogs often gets supplied with a Kg increment dose printed on it, however heavy the dog is - thats the dose that is given, its just easier for people to understand & work out for dosages with dogs.
however the kg dosage method for guineas is a bit confusing & not as accurate. hence vets usually give dosages in ml.
the baytril is an antibiotic & will help to kill off any infection. although if the lump turns out to be an abscess then it may not be strong enough to clear it up & 'Zithromax' is often used as the 'big guns' to help clear the pus/infection (this is unlicensed on guineas-so you may need to sign a disclaimer for its use.) quite a few members on here have had success with Zithromax.
You should give the probiotic (avipro/pro-c/biolapis) about 2hrs after the Baytril dosage - if you give it with/before/immediately after wards the antibiotic kills off the good bacteria as well as the bad & so it won't be of any use, it must be given 1.5-2hrs after the antibiotic (baytril).
try & sprinkle powders on top of their favorite food & handfeed or add to a small 1ml syringe of water to syringe feed, that way you know how much they are getting ( if you just add it to their water bottle it may taint the flavour so that they drink less water or if you have multiple piggies, you don't know how much the poorly one is getting.)
Baytril is quite harsh on their gut, that's why a probiotic is recommended (they usually hate the taste of baytril too!)
Are you sure the vet gave you steroids? what was it called? Guineas are not generally supposed to have steroids so i would double check with the vet if they have prescribed them. ask for an alternative if possible.
did the vet try draining any fluid out of the lump, possible abscess with a needle? has the lump grown quickly in size? please keep an eye on it & take back to the vets if it grows or bursts (if its an abscess it will stink) it will probably either need to be removed or drained & flushed through to get rid of any pus.
if its a dental abscess keep a note of how much the piggy eats-as they can easily cause dental deformities if the teeth aren't being ground down with hay eating, due to pain making piggies lose their appetite too.
daily weighing will help whilst she is unwell, just to make sure she is gaining weight healthily & gradually being so young.
fingers crossed & healing vibes being sent for your little one.xx