due date calculation ?

I just found out we 're expecting baby number 3 . It s scary as our 2nd baby is only 7 moths old but we 're exited non the less. Now here is my question. Because I am exclusively breastfeeding I have irregular Af. The last I had was back in October (I think) and it was an unusual 2 day bleeding. so how do I know when I m due or how far along I am? I 've been feeling flu ish for a while (nauseous everyt ime I eat) that why I poas today but even that I can't remember when I started feeling this way.
Any other way to find out a due date apart from an u/s ?? Tia

You can calculate from that as if it were a period.
Or start from the one before it.

Other than that you're going to have to wait for a dating scan at your dr, it's the only sure method.