Interesting how the doses vary depending on the source... The horse dose listed in Plumb's is closer to what I have use when I've used it on horses. One of the horse doses listed in Plumbs is 20,000iu to 44,000 iu *twice* a day, which is quite a bit more than what drugs.com suggests.
Not saying...
Based on what I just read I don't think you gave too much. If anything, you gave too little, but please do check my math.
Here is some info:
Picture below is from this site: http://smartools.pharmdev.org/iu.html
Label on bottle works out to 13.2mg/kg and there are 300mg of penicillin...
You can get it from Tractor Supply. If the penicillin doesn't work you could try Baytril, but it's banned for use in poultry, so research that before you buy it.
Post 35 has penicillin dosing info...