This probably has to do with how you are taking your Valsalva. Focus on directing the pressure from your breath down into your abs and not up into your head. You may need to practice this unloaded to get a feel for it going in both directions. In other words, try to take a big breath and make your head hurt with out lifting anything. Then try to take a big breath and direct the pressure down to not make your head hurt. If you can do this then you can also do it when your lifting, it just may take some practice.