No period but not pregnant?

So one of the weirdest things happened to me, where I didn't have my period at all.
​My fiancée and I freaked out and got a pregnancy test but it came out negative.
​So how else could I entirely miss my period? I will admit that there was some blood but not nearly enough to be my period (based off of previous periods).
​Also, what should I do? I'm not sure if I should take another pregnancy test or just go to my doctor?