Who to call to check for carbon monoxide?

Lately i've been worried tht the home I live in is leakin CO from the fireplace.. It's a gas fire and this house is extremely old... I'm pregnant so i could just be overreacting and it's pregnancy symptoms but lately i've been gettin sooo dizzy and headaches... Esp today and the fireplace is on.. I usually dnt get like this until it is on.. I live with my bfs grandmother and i'm sure shed think i was crazy... but id rather be safe thn sorry... Who would I call to have it checked.. And would it cost..?