FWIW, prop gives me test flu throughout the whole cycle (i usually only run prop 8-10 weeks tops)
Some of us really just can't build a tolerance for it, but it still works. Like you said, antihistamines do work, and when it gets bad will get me through training sessions. You have to remember your levels are slightly rising within the first week and a half. So they are peaking now and the symptoms will be there. If it feels like lethargy, fever, head cold, its prop flu. You can push through it, and actually most of the time getting in the gym has helped me kick some of that feeling.
Sometimes its not even the BA. I think something like 70% of the general population just doesn't react well to the propionate ester. The way it binds is abnormal and causes histamines to build up somehow. I would find the study done, but its been a while since I read it.
Just keep an eye on shit, no harm in being careful. For now I would bump it down to 75-100mg EOD. A little prop goes a long way. I gained 23lbs off an 8 week Prop cycle at only 75mg EOD with a dbol kicker. I kept 12 of those lbs too.