Will motrin or regular aspirin stop or slow down a period?

No: Aspirin and motrin help with menstrual pain and symptoms .They donot affect the amount of bleeding

Platelet inhibition: NSAIDs can help relieve discomfort during menses, but since they inhibit platelet aggregation, can increase the amount of bleeding during a cycle. This effect should be minimal. These medications should not stop a cycle or make it shorter.