Neck pain after RCT, worsens at nights. Could it be due to RCT or sinus infection ?

I am 43 (Female) 145 lbs, 5 8. Never had neck pain before, but it would seem that about 2 days after a root canal over a week ago, I can barely move my head when first getting up. (the pain has woken me waaay early (2 & 3 AM) for the last two nights-not able to sleep afterwards. It does get better as the day progresses, but then gets bad again at night, and after I ve slept for a few hours. I have not injured myself - I m flabbergasted as to what could be causing this! (Root canal??) It was performed on my bottom left 1st molar. I went to an acute care cntr 3 day after it s onset, she gave me Flexeral and antibiotics for a stubborn sinus infection . (could it be from the sinus infection?? Finished the 5 days of meds for that - still feels like a bit stuffy/uncomfortable) The Flexeral made me sleepy, and I can t take them in the day. They really weren t doing much for me at night either. Too many variables?