This happened to me once; when I went to A&E, they said it was probably (but could not be sure) that a small chocolate cyst had burst and/or the pressure of the penis penetrating had 'dislodged' the endometrium and kick started my period. I don't see why or how it could delay the surgery (I've had two when on periods).
If it continues and/or you're not normally due on maybe pop to your GP and see what they think.
They won't delay your surgery if you have your period. In fact there may even be a benefit, as your endometriosis will be at it's most obvious.
If you have a lot of pain, or you are concerned, it's worth seeing your GP for a check over, but it is quite common to have spotting or breakthrough bleeding on various hormones based meds, so could be that.
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