Hello, I appreciate your concern about your baby and i would like to tell you that you be relaxed and take a chill pill, because everything is normal. Placenta is posterior and its absolutely normal, so you need not to worry. if placenta previa is there, then you need to be concerned. Low lying placenta is there and thats again very common and you need not to be worried much about this, just take few precautions like dont carry heavy objects, avoid sexual intercourse, avoid long traveling by road. But you can continue your yoga safely. even you can do regular walking also. Traveling by train is also safe, but you should avoid it after 8th month. Infact, good thing about low lying placenta is that there are more chances that you will have normal vaginal delivery rather than LSCS. Hope this will help you and best of luck and best wishes for your pregnancy.