What Causes Heavy Vaginal Bleeding Along With Blood Clots During Periods?

I am a 36 year old female who has been diagnosed with uterine fibroids and had a myomectomy about 3 years ago. It is not unusual for my menstrual cycle to last 3 to 4 weeks and pass blood clots for the duration of the cycle. For the past 2 weeks I have been passing menstrual blood clots that seem much larger than usual...some larger than the size of about 3 tampons held together. Should I be alarmed since the blood clots have gotten considerably larger?