pregnant to soon?

I have 2 boys my eldest is 4 1/2 and my youngest is just over 2 months, I have been ill since giving birth with D&V; and all sorts so been pretty much home bound. I stupidly had unprotected sex with my partner back in December after my 1st normal period after birth. I'm due on between the 12th and 15th January..I'm not sure on the exact date. but I have been feel sick and constant headaches also I have been sleeping for majority of the days. I have also had cystitis which all these symptoms I have had with my previous pregnancys. I know your the most fertile after pregnancy and I am never this silly not using contraception but being so ill and running tests at doctors I also blame baby brain! but 2 kids under 1 is not what I had in mind. any one else been in this situation?