Testosterone propionate itself, in a non-objectionable carrier, is absolutely painless to inject. There’s nothing inherently painful about it.
But due to the fairly low natural solubility of TP in vegetable oil – about 100 mg/mL – there have been a lot of preparations made that are high in benzyl alcohol and thus are painful.
plasticman makes a good point that’s hardly ever raised but should be from time to time, that it’s in fact possible to be allergic to a given vegetable oil. For a homebrew Wesson (the standard variety) seems as nonobjectionable as possible. I suppose it’s still not our of the question, some could have an allergic reaction even to it but the likelihood would have to be way less than for example with sesame or peanut.
Hopefully the heat and swelling problem isn’t still the case?