11 days late and irregular periods

TTC for a few months now and my periods have been on a 31-33 days cycle. March 1st, I had my period as usual, but then it came again 21 days later from start date. Then, no period in April. So, am about 11 days late if I go by my last period starting about March 21st. I took two First Response tests about four days ago—both negative. My friend says to wait and test again when its been 6 weeks since last period. I am trying to wait til this weekend.

If still negative, I guess its time to see my doctor... I am getting so anxious!!