Pupils different sizes, responds to light?

Two weeks ago I realized that one of pupils was significantly larger than the other. Knowing this is sometimes a bad sign I reported to the ER to get checked as the night wore on a developed a light headache. The CT scan was clean, and a chest X-ray I had 2 months previous was also clean. The doctor stated it was most likely a medicine response or just happened as it sometimes does. For the last three days I have developed a very light headache on one side of my head, and my pupils will occasionally be different in dim light. They do, however, respond properly to light and constrict. I was worried about Horner's syndrome since it can be caused by lung cancer, but since the chest X-ray and head CT were clean it isn't likely. My neck has been popping ALOT over the last 3-4 months, with me feeling like I need to pop it several times a day, and I usually do. Should I be worried? Should I get the MRI/MRA? My eye doctor said my eyes seemed to be working fine the day after my ER visit. Im just really worried that is all.