Pregnancy changes things, including fertility issues. So many women have trouble at first and then have no problem the second time. If you're having unprotected sex, yes, absolutely you could be pregnant.
Please talk to your doctor immediately - if you are pregnant, you should be nourishing yourself accordingly and avoiding the usual risky things. Your body still hasn't recovered from your daughter's birth, so if you are pregnant you may need extra care or supplements. If you're not pregnant, you may wish to rethink your decision about no birth control. You can also do a blood test which is much more accurate than home pregnancy tests. With all your shifting of hormones and body changes, anything could be going on. I'd stop spending money on the home tests and get with a qualified professional asap.
Good luck.