I have been pegnant 9 times and have 6 children. 3 of my pregnancies happened less than 3 months after giving birth and one was 3 weeks after baby was born.(Nurse gave husband bad advise. lol). Each time I used a different type of birth control and each time I was nursing my baby 100% of the time and every 3 hours.
I found my periods were different with each child. Some came right away, some waited until after I was finished nursing. Some the first period was - heavy, others were light and one I only spotted.
My suggestion to you is don't worry.... if you are pregnant, you will find out in a few weeks (home preg tests may or may not work) - there isn't much you can do to change the fact right now and your family doesn't need you to worry. Enjoy being a mom and wife, having fun and as for the nausia - take it one day at a time. Let you husband help out more until you feel better and treat it as the flu. You might even get more sympathy that way. lol.
If you are pregnany, it will be hard at first but the kids will be closer in age and hopefully best friends. Just think, they will grow up and be out of the house sooner. :) Believe me... it happens all to soon so enjoy every minute.
Good luck in what ever the future brings.