If your wife is negative then nothing to worry ,your wife and baby will not be affected.
Till your treatment is complete and you become negative you should avoid sexual contact with wife or use barrier contraceptives (condoms)
The very first thing you need to do is, go for a scan. Because she must be having some infection related symptoms. Abnormal white discharge, pain, nausea and back pain may be result of PID, so go for an ultrasound. And stop taking I pills daily, this is emergency pill. If someone is taking regularly, it may cause some harm. As hormonal imbalance. Meet a good gynaecologist near by. And choose better option for contraception.
MD - Obstetrtics & Gynaecology, FCPS, DGO, Diploma of the Faculty of Family Planning (DFFP)
If you did not have sex after last normal usual period and after withdrawal bleeding followed by unwanted 72 then no chance of pregnancy. If in doubt get blood test done serum HCG-Beta or meet Gynecologist.
If your LMP is 18/5, then you may have the next on 15/6, if it goes on regular 28D cycle. Many have deviations from it, may have sometimes advance, sometimes delayed.
Nothing to worry, wait till 1 wk after 15th, then think of other ways.