What Causes Headache After Using Vacuum Helmet?

I have a brain related question. I'm using a vacuum helmet (an airtight helmet attached to a vacuum cleaner's nozzle) for increasing bloodflow to the scalp for promoting hair regrowth on bald head. I setup the helmet myself after reading a patent that described it. My concern is, will the increased bloodflow cause high Intra cranial pressure and damage the brain? I do take 5 minute intervals for every 15 minutes I use the helmet. If I leave the helmet on continously for more than an hour, then I have noticed it causes a headache (but disappears within minutes), that's why I got the doubt about possible high ICP from using the helmet.