I am not sure if you are in the U.S or not, but there is a huge flu epidemic going around. You may be coming down with the seasonal flu or have an upper respiratory infection. It sounds like it started out as a common cold and progressively went to a high fever. I do not think that it is a carry over from your sinus infection, especially since it has been over a month since you got over the sinus infection. It is probably a new bug.
My advice is try taking something over the counter, like theraflu or nyquil. If it does not get any better, you should go to the doctor.
Also, if you work in a place with lots a people or have contact with lots of people daily, you maybe picking up these bugs from there.
Once you feel better, try to buying some hand sanitizer and wash your hands more frequently than you normally do, especially when touching doors, etc.
I hope you feel better!
Yes, I dont think this is due to the sinus infection since it has been so long and the chills and fever wouldnt be apart of that either. The yellow that comes out of your nose when you blow does however mean that you could possibly be getting another sinus infection. it sounds like you have just caught another virus. I know Its crazy but there is alot of viruses going arround right now its possible you just got over one and caught another :( Feel better