When we are stressed we can skip periods or they can go haywire timings wise, so if you rule out pregnancy for definite then put it down to stress. If it's still not here in a fortnight test again. By then if you are pregnant it should be a clearer result. If you come on in the meantime then you know it's the stress.
With a bit of luck you'll be on your period for your op and the surgeon can clearly see all the endo spots being active and attack them. It's much harder to see them when you are not bleeding.
If you're feeling tired you could be run down and picked up a virus or bug, plus it is hayfever season too, so if your body is fighting off the effects of pollen then you will be knackered while that is happening.
Worrying about a late or missed period plus the op itself will just be adding to your stresses.
If you are definitely not pregnant, then don't risk getting preggers before the op. Use condoms or abstain or alternatives, so as not to add to your pre-op stresses.
Please take timeout to relax and rest.