It could also be an infected hair follicle or other type of abcess (these can occur at any time - no matter how much saddle time you have in). Sometimes they can be just hard little (or big!) lumps for a time until they finally come to a head to drain. Sometimes they are painful/warm to the touch - sometimes not. Not a nice option - but you can usually treat them at home and unless they are really nasty they don't require surgery.