Your doctor knows what she/he is talking about.
Did they give you a blood test, or a urine test?
If you had a negative result for a blood test,
.I really highly doubt you're pregnant.
He's right, if you were 3 months pregnant,
.I'm pretty sure you'd be able to find out.
Just try to relax &stop worrying.
Easier said than done, but just take it easy.