Quads can be finicky..There easy to pin, but if you use a compound that's none to have a bit of pip, it really tends to get over exaggerated in the quads...
The compound could be sitting on/near a nerve agitating the local area,at the same time getting a slight immune response from the lymphatic system..This also can be induced from the bb/ba agitating the areas mentions above.
Now Aside from the BA/BB... The ester in your NPP is prop, Prop is taken from proponic acid, thus its an other reason why people tend to get more of site irritation..Also keep in mind the short ester chain (3 carbon/C3)..Lower the ester chain, more of an immune response you'll get.. Here's some food for thought; some bug bites,especially bee stings/venom is made of C1 (one carbon)..Prop isn't that far away..With this said, solvents and low carbon esters have a tendency to trigger more of a immune response..
With the combination of the solvents and acid nature of the ester, this is what will cause site irritation with the increase presence of histamines in the general area, causing localized swelling ( immune response ).. This is why Antihistamines tend to assist with post site injection pain/swelling!
End result; Short esters can be a pain, literally!
