I think you may be right to consider that there is somebody else... sadly all of his behaviour does point to somebody else being on the scene. He is being nasty because you keeping the baby is going to "ruin his life". that is the usual story....
Is there a school bus that your kids could get on to? You could contact the local council and ask them.
Are you on any medication for the SPD? I didnt like to take too much, but the doctor reassured me that Paracetamol and CoDydramol were both ok for the baby. SPD tips, keep your legs together at all times, as much as possible, getting in and out of cars etc.... Sleep with as many pillows as you can to support your body in a comfortable position. But I know how bad it is, I used to cry with pain every day nearer the end and just wanted the baby out... Can you use crutches to get around at all? Have you seen a physio? some can be really helpful with SPD.
Regarding the ex, try and stay detached from him as much as possible. Don't let him get to you, don't let him take anything, if he wants it he can buy it for himself now. Contact the tax credit office, the council tax office, tell them that you are on your own now. I am not sure whether to suggest it or not, but if you really have nobody else to help you, would social services be able to help? You will have to go into hospital at some point and you will need to know that your children are being looked after.
Your ex has a duty to maintain his children. if he wont pay you, then get the CSA involved asap.
I would also box up all his stuff and leave it outside and tell him the time and day that you are going to do so and that he is responsible for collecting it. It is not your problem if he has nowhere to store it...... He needs to start taking responsibility for his own actions.....
Please look after yourself and keep posting here, you will get good support..