Vesatot are generally calculated from the onset of the flow, and not from the stain on the bedikah. Therefore, you should calculate next month's veset from the flow on Thursday.

If however, for three consecutive cycles you become niddah from a stain on your bedikah on your onah day, even if it precedes your actual flow, you would then start calculating your veset based on the bedikah and not the onset of the flow. If this does occur, you should make sure that you are actually becoming niddah from the stain on the bedikah. If it is not bright red, you can bring the bedikah cloth to a halachic authority for evaluation, since not all shades of brown will render you niddah.

Please feel free to get back to us with any further questions.