hello..... stop the presses!! you may or may not be beginning the stages of menopause and only a blood test for your hormones can truly indicate as such.. HOWEVER... even "if" you were beginning peri-menopause...... as long as you are still ovulating, there is still a chance of becoming pregnant as will all women tell you especially those whom at this time are in their 40s and trying to conceive.... With that said, your doctor may tell you that it does become harder to become pregnant in your 40s... which of course, you are probably already aware. Periods do change throughout one's lifetime and could be that yours are in fact changing and yes, things like stress can affect it.. I would suggest a couple of things, one DO get those blood tests... two, once you stop breastfeeding, DO chart your cycles to truly find out IF you are ovulating... and this way, you can get a better and bigger picture of what's going on from month to month with your cycles. A good book is , " Take Charge of Your Fertility" it's good even IF one isn't trying to become pregnant in that I think some women (not all) think the period is the main event, when in fact it's merely only part of it... especially when trying to conceive as you begin to learn that ovulation is VERY important and or progesterone levels and estrogen....
Lastly, now, this is just my own personal opinion.. but IF you are considering having another child.. start now........................ not sure if you conceived your first one easily and hence there could be this thinking that it will happen easily again and not to discourage you, but .... it doesn't always happen as easily on the second try... I say.. start now.... of course, see what your doctor says.. Also, there are many websites out there these days filled with women in their 40s who are or have conceived.. one of which is fertileover40... many of those women have been told.. you hit menopause.. don't even try.. or your too old, your chances are slim to none.. but those women have proven the doctors and naysayers wrong..
try and remain positive and realistic...... the both come in hand ...
best of luck to you