This is very normal and is actually good for you, indicating increased blood flow to the brain, eyes, skin and hair. One preliminary study showed that the brain runs 7% faster and 14% more accurate while inverted! The feeling of pressure usually lessens over time as you become accustomed to inverting.
If you are a beginner and are uncomfortable with this feeling, it is OK to come up and rest a while. This is referred to as "intermittent" traction (alternating inversion with being upright) and is a good way to help get used to the inverted world. You can also try "oscillation" which is a rhythmic rocking back and forth.