I'll fly for 20 hrs and i was planning to take aspirin against blood cloths, but i'm afraid it would make my flow heavy. what should i do?

Thromboses: I cannot see that the small amount of ASA would cause menorrhagia. Even better, get up and walk the aisle every hour. If the flight is empty, elevate you legs on empty seats. Avoid crossing your legs during the flight. If you have a history of problems, but supportive hose.

Walk around.: Your best bet on a flight like that would be to get up and walk around - say every thirty minutes or so. Just get up from the seat and walk the aisles for a minute or two. Truth be told, aspirin use may not even be strong enough to prevent a blood clot from forming if you're immobile for twenty hours.