You are suffering from delayed period with H/O chicken pox
3 wk ago and did sex 2 wk ago.
Do repeat pregnancy test with blood for hcg as condom has 14-15% failure rate.
Chicken pox may interfere with delayed period and beside this, other reasons are stress, extreme wt loss, eating disorder
, hormonal imbalance, ovarian cyst
, thyroid disorder
So consult with your gynecologist and need some investigation like USG lower abdomen, some hormone tests etc.
Take healthy diet rich in protein, avoid stress by regular deep breathing and sleep properly.