First of all you have to control your conition of hyperthyroidism
first. It is important to keep the thyroid level under control before you plan to concieve again.
Regarding the second premature delivery, it is not the position of the fetus which can cause this because of walking. Doing normal household work and walking is not going to do any thing in case of normal pregnancy
like what happened to you. It may be because of some other reason, so keep in mine that walking or doing normal house hold work is perfectly OK in pregnacy until there is some complications.
Now the present case of not having the periods is may be because of some hormonal
disbalance and usually after delivery it take 2-3 months to regularise this also. The drug meprate
which is given to you is only a hormonal replacement and it is not a drug which will start the periods. Another important thing is that it may be becasue of the weight loss
and your condition of hyperthyroidism which may be interfering with the periods. I would suggest you to get a proper check up for you thyroid profile and try to keep this under control and also keep in mind that hyperthyroidism is a chronic condition and requires long term and continous treatment.
Thanks and take care.