Will I be committing a sin if I am in the Iddah (divorced waiting period) and continue with my Dr's apts.? My husband does not have a house or apt. I either stay at my parent's house or my brothers' house which is near the hospital. I do wear full Hijaab, no makeup and do not go out for fun or parties, only when it is extremely necessary.
Should I continue talking to my husband on the phone because I do love him? But he has repeatedly told me he would not take me back. Also do I have to wait 3 menstrual cycles because of my illness, my period is irregular sometimes 2-3 months apart.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

We have previously issued many Fataawa about a woman in her waiting period, so please refer to Fataawa: 89688, 85920 and 86515.

If the divorced woman is not pregnant and has menses, then she should observe three complete menstruations as Allaah says (which means): {Divorced women remain in waiting [i.e. do not remarry] for three periods.}[Qur'an 2:228], so you have to observe three complete menstruations. When you become pure after the third menstruation, then your waiting period has expired, but you should not count a menstruation in which he divorced you (if he divorced you while you were in menstruation). Your menses being irregular does not constitute a problem since you have menses and you become pure after having the menses.  

Allaah knows best.