It could be, but in my expert opinion (okay, so I'm not really an expert, but it sounded good, alright?) it's unlikely. You could be adjusting to the changes, like you believe or you could be dealing with something else.
Firstly, I'd ask your doctor if he thinks you could possibly be anemic. A lot of what you've said reminds me of myself a few months before I got pregnant, and that's what was wrong with me....iron deficiency anemia. I got on some iron supplements and started paying attention to what I was eating and started feeling much better. (Paying attention to what you're eating can help in more ways than just extra iron too, fruits & veggies have vitamins & nutrients in them that can help perk up a mood way better than a "happy pill" that the doctor could put you on!)
There is also the possibility that you are suffering from depression. If you are, then it's important to know the difference between postpartum depression (PPD) and clinical depression (because one will pass and won't leave you on meds forever, whereas one potentially could).
PPD is depression caused by a hormone imbalance. Hormones...you remember those right? The nasty cravings, the mood swings, the swollen..well...everything? That was hormones. And believe it or not, 10 months after everything, you're still not 100% back to normal! Especially if you're breastfeeding. Eventually though, these hormones will return to their normal levels, and will stop causing you problems. If your problems persist though, then you likely have clinical or "regular" depression.
Clinical depression is caused by a Chemical imbalance in the brain. Generally, this is what medications help with...they rebalance your brain so you can feel normal again. They don't make you happy, they make you feel reality, happy or sad. If they make you happy all the time (or make you stop caring about everything) then either you have a really happy & awesome life and nothing ever goes wrong, or they aren't the right medication for you. Sometimes it takes a few tries to get the right med & the proper dosage worked out. Of course, rather than jump straight into meds, most people prefer to try some form of therapy or counseling first. This can help in a number of ways that meds can't, AND it's not something that will make you sick if you skip a "dose."
Good luck, and no matter what your diagnosis, don't be afraid of the treatment. It's for your own good and your kids!