pressure in head that builds then releases waves ?

I am 33 and have breast cancer stage 2B which I have been all clear for almost a year now. For almost the past 2 weeks I have been getting this weird building of pressure that builds up from anywhere between 2 minutes to6 minutes or so then releases and leaves me light headed. This happens in continuous cycles all day and night. I would say this happens hundreds of times a day now. Very faint dull pain say 2-3 on a scale. Had Ct (without contrast) and blood work today in the ER. They said everything looked good and to see my Oncologist tomorrow and he may send me to neurologist. No sinus isues ever in my life and no symptoms on CT or in general that would be sinus related.
Any ideas or anyone experience this same thing?

Is a non contrast CT a scan that I can be comfortable with seeings how I already had cancer and my chances of spread are high? They said all clear, but now I'm wondering if I can take that as a good enough answer. I can't have MRI due to breast expanders. Wonder if I can have a PET scan.