I've heard you're not really supposed to take any of the DM versions. You can ask your pharmacist, or just call your OB and ask.
Im 35 weeks as well and started with a bad cold and mucus. Im not taking any meds but my Dr told me to drink Robitussin Dm.
My OB says Mucinex (the regular kind, not DM or anything) is okay, but a lot of women try to tough it out with honey and humidifiers. Sorry you're feeling so bad, being pregnant and sick is not fun. :(