Well done! You’re quite right - if you keep putting off the “nasty thing”, it will just get bigger in your mind, and you’ll end up never going at all. Just think: by Tuesday evening, it will be history! Plan a treat or something, when you get home, to reward yourself for going.
Unless you know you are anaemic, it will make no difference at all if you are having/have just had your period.
I take it, from what you say, that you’ve never had a blood test before?
Neither had I, 'til I was forty-something, but since then I’ve had so many I’ve lost count! Never, not once, have I ever been asked about my period, or advised I should postpone the test to avoid it. So it’s not relevant, at all.
You seem to believe you will feel “very poorly” after the test. The only thing that’s going to make you feel poorly is anxiety. The test itself is not sufficient blood loss to make you unwell, so the hurdle is psychological.
It’s similar to an injection, but just a little bit longer the needle is there. Moments, but NOT minutes.
Just don’t watch, and tell the nurse you’re nervous. She will probably talk to you and distract you. You’ll feel the needle go in, but if you get a really good one, you won’t even notice she’s finished, and the first you’ll know about it is she’ll ask you to hold a bit of cotton wool on it while she gets you a plaster.
You don’t need the rest of the day off work, or to rest up, or anything. You can carry on with whatever else you had planned.
You’ll be so pleased you had it done. I shouldn’t think anyone actually likes these things, but there are times in life when you realise it’s necessary. When you’re ill and need answers is one of them. You can’t just go through life not knowing what’s wrong.