What you are seeing is an estrogen rebound from stopping arimidex and ther still being test c in your system.
Layman's Rule of thumb with all drugs is that it takes about 4 half lives to completely clear your system. Cyp Has an 8 day half life.
These following calculations are for arguments sake to illustrate a point, lets say you were using 1000mg cyp once a week and to make the calculations easy, we will use a 7 day half life instead of 8
Immediately after your last Shot, There is 1000mg + 500mg (from week 7) +250mg (from week 6) +125mg( from week 5) totaling 1875 cyp floating around. One week after your last shot Its 500+250 +125 for a total of 875 mg. 2 weeks after its 250+125, 3 weeks after its roughly 125, 4 weeks it's pretty much gone.
This is where it becomes kind of a bitch to calculate the optimal time to start pct with long esters. Most will say 2-3 weeks after your last shot, as you don't need to completely clear your system to start.
I have never personally come off on a long ester, I always use an oral for the last month and start pct the next day.
*edit* i'll pass the buck on this one to someone with more experience coming off the long esters.
Aslo, how long ago was your last shot?