Fleas problem Help?This the 2nd time has happened to me.?

This happened 4 weeks ago.I've get rid of my curtains to be washed and vacuumed my carpet after an exterminator had sprayed my whole house except my parents bedroom.Everything was fine after a few weeks.But today I found them on my feet on the living room,which is just in front of my parents bedroom.After that,I saw a flea on my mother's shirt in her bedroom.I don't have pets,but i do have stray cats staying outside of my house.That is maybe the reason,but what should I do?Call an exterminator again to spray my parent's bedroom or...?Oh and my mother is a naive person that things that by vacuuming her bedroom has killed the fleas.