If my ovulation test was positive then why would I be late for a period if a pregnancy test is neg?

Probably OK. So predictable is the menses with many women that any time there is a change - timing. Duration, consistency, color, for ex. - red flags are raised - and appropriately so. It's smart to ask about this. Many women will experience, from time to time, some alteration in menses character. Freq this resolves. If persistent or signs of pregnancy, it may be a good time to see your dr.