Without pain or other symptoms like heavy bleeding, it's probably nothing to be concerned about. But any change in your normal cycle should mean seeing a doctor.
I passed clots for 20 years without pain, my doctor said it wasn't a problem. The only thing that brought relief was bcp, but those brought their own problems. Without a foul odor or fever, it's probably nothing bad. It could be polyps or fibroids. Polyps can be treated with a d&c, fibroids can be removed surgically.
Personally, it sounds to me like polyps. My doctor also suggested oil of evening primrose.
Give your doctor a call and make an appointment. Let us know how it turns out.
Take care.