What Causes Delayed Periods?

ok so im pretty irregular, so i have 3different phone apps to help me determine my period. Each say im 32 to 34 days between each period. Well my last period was dec 29, which lasted 3days. Thats weird though cause i always last 7days. Well now my apps say i was suppose to star either Jan 29, Feb 3, And Feb 7. And i have yet not started. I took a Pregnancy test for the fun of it, but it came out negative. I dont know if its cause i drank alot of water right before, which cause my pee to be totally clear. So what do you think is going on, am i just late or can i be pregnant?