1) First of all don't disclose to any one in your current organization that you have got new job, not to your colleague's or best friends as well strictly prohibited.
2) When you have given resignation letter on medical ground now you submit on more medical certificate in your current organization that one immediate basis your doctor has asked you to take bed rest for atleast 3-6 months and not go to work out of the home.
3) So take relieving letter from HR that you have leave company on immediately and want proper reliving letter plus experience certificate and PF Plus leave benefits.
4) Make all above points in your letter and get signed by HR along with company stamp seal.
5) Still they do not listen to you than tell them that you are going to complaints against company with registrar.