What Causes Delayed Periods?

hey im 18 years old and have a little problem i came on my period on the 16th of march but was going on a night out on the 19th i bled on the 16th and 17th then found northistarone tablets in my mums draw and read the directions for use and how many to take so on the 18th and 19th i had 2 tablets on each day one in the morning and one at night i was still on a light period then stopped on the 19th so all was well for me i then came back on but light on the 21st for a further 3-4 days now from my calculations counting from the 16th my start date i should have come on my period on the 13th of april and my period has still not arrived have these tablets messed up my cycle for this month of could i be pregnant although i have done 4 pregnancy test and all were negative unless my hcg levels are low