A lighter or different-feeling flow like this is a change some women notice after starting metformin and inositol together, it doesn't by itself mean something is wrong with your cycle.
Next Steps
Since your letrozole timing depends on this genuinely being day 1 of a cycle, please call your treating doctor today before tomorrow's dose so they can confirm that with you directly.
Health Tips
Please don't guess on this one, timing genuinely affects how well letrozole works that cycle.