Is your daughter able to wear internal protection i.e. Tampax ? I was never able to. I had to wear several pads at once under 2 pairs of knickers, as I would leak/flood. Night times I would lay on a thick towel to protect the bed, again with 2 pads. These needed changing every 2 hours. When I stood up the blood would 'flood' out ........ fortunately the protection in general, caught the flow.
She is 18. An adult. Despite her OCD she will go to the GP if she feels it necessary, or she can get a phone-back by her GP. At her age this is between the GP and herself.
Maybe you need to discover why you react in this way? Is blood alien to you? How were you raised to 'see' monthly bleeds?