What Causes Fatigue, Dizziness, Headaches And Delayed Periods With Light Spotting?

hi, i have been experiencing fatigue, dizzy spells and headaches for the past week. my period was due to arrive on the 4th march but it didn't. i was at the doctor on the 2nd of march to get penicillin for a throat infection. on the 11th march i had very light bleeding for a couple of hours but nothing since. i took 3 pregnancy tests and all were negative.