I have had an older cow, appear to be bulling about a month before she was due to calve. I cannot remember now if it was blood or slime or both that she produced, but I distinctly remember worrying that after all this time she now appeared not to be pregnant. I did nothing about it and then a month later she calved as normal. So false heats do happen in the course of normal pregnancies.
I would agree - see what happens in 3 weeks time.