Why do they think im lying?

My ex fiances mom told my mom I am pregnant after I found out im not (because I started my period) I told both of them im not but his mom believes im lieing because I've been moody and gaind over 10 pounds in a couple weeks. So my mom made an appointment..but why do they think im lieing to them

that's tough but it could be a thousand reasons why they think your lieing sry I couldn't be more of a help