you're most likely just one of the lucky ones. i get really sore and sometimes a little swollen the day or so after i inject prop. then i get a knot for days after. my training partner has barely any side effects from it at all. i know a lot of people that it doensn't bother and some who react to it like i do.
it's not the ba% that you're using either.....i've used up to 5% in test e and hade very little pain. from what i understand, and someone correct me if i'm wrong, because the propionate ester is more difficult to keep suspended then other esters it crashes very soon after being injected. meaning that it drops out of suspension and re-crystalizes once injected into the muscle. and this is what causes your pain. similar to suspensions like test or winny.