6 month old having nightmares

knottie170195ea3f929846

Once or twice a week my 6 month old daughter will whimper in her sleep and start crying very softly. Usually I pick her up and she still whimpers but eventually she settles down while still sleeping. Last night she started crying out loud while she was sleeping. I held her and calmed her down but I'm getting worried. Can she being have nightmares? Could teething cause this? She drools but I haven't seen any other sign yet for teeth.