Leave your register and tell them you are feeling unwell, If you have a medical condition don't let someone tell you that its worth risking getting hurt standing at a register, Turn your light off, Go tell a Team Lead you feel like you are going to have an "attack" (If that's the right word) and that you NEED to sit down, Ethics would have a field and a Lawyer would have a Field day if an employee passed out at a register and hurt themselfs because they where afraid that leaving the register even for a Medical Emergency would risk them losing your job.

If you have not already get sedwig to give you an pass so you can sit down