I wouldn't tell her anything. When you meet her just say you woke up this morning feeling like shit. It's not lying.
If she's into you it wont even matter. I've pushed girls away from me telling them that they will catch my cold, but they can never resist

I think it actually acts as a kind of push pull attraction.
(Lean in to kiss HB)
Me: You shouldn't kiss me because i'm sick
(Pull away)
HB: I don't care if I catch your cold
(Lean in to kiss HB)
Me: This is a really bad cold
(Pull away)
HB: I don't care
(End up kissing, undress, ect...)
You'll be fine. Your thinking too much. Telling her not to come to see you would be a total let down for her. Just go with the original plans.