I was 5 months pregnant and still had my period?

Today i just lost a baby and i didnt know i was pregnant, i had my period normally, and my belly didnt grow. I had no nausea or anything. I just felt like i had to go to the bathroom and then there he was. But we didnt make it on time and my baby boy died 5 minutes later. But 2 weeks ago i donated blood for my husband so how come they didnt tell me i was pregnant when they tested my blood? And why i never felt my baby moving?