Uterine lining 7mm day 3

Wendy • Expecting baby #1 11/8 after ttc for four years
I had an ultrasound done this morning on cycle day 3 and my lining was 7mm. I have had 2 very heavy days so I was surprised! They had me do bloodwork to make sure I wasn't already pregnant that was negative. Is   It bad to have it be this thick this early?