thank you for your query.
A negative blood and urine pregnancy
test rules out possibility of pregnancy, unless your cycles are very erratic, and you had delayed conception. Possibility is remote.
Also, thickened endometrium
means endometrial hyperplasia
It could be due to hormonal imbalance, age related, effect of hormonal therapy or a precursor rarely of cancer of the uterus.
Go ahead with a confirmatory scan, and if required, a diagostic curettage alongwith hysteroscopy