Generally, the "test flu" is far more common with short ester preparations like propionate.
As a rule of thumb, the shorter the carboxylic acid chain, the greater degree of possible tissue irritation that it can induce. I mean, that's what these esters are - acids - so they can cause a localized reaction that the body will react to in a typical defensive fashion.
I, for one, have a very strong reaction to the proponic acid ester so I can't use any prop whatsoever.
Back in the days when I was competing, a single shot of prop would cause a big red welt on my butt the size of a golf ball. The welt was warm to the touch and for 24-36 hours after each and every shot I felt like I was on the verge of getting sick - low grade generalize fever, aches, tired.
I've never had that problem with either ent. or cyp. however, that doesn't preclude the possibility.
As Chip suggested, try switching to ent. if you are currently using cyp.
Regards.