I weigh 107 pounds and I am 3 days late on my period I took tqo pregnacy tests bothe were negative what could b a logical explanation for this?

Several possibilitie. You may be too skinny and this can stop periods sometimes. Also stress can do this. Other hormonal imbalances can play a role. If you skip another period, see a doctor for further evaluation. Also, do another pregenancy test in a wk or two.