When I was 34, I noticed my periods got much lighter and went to the GP about it. She ordered standard blood tests where they check hormone levels on days 3 and 21 of cycle and they came back normal, indicating I was ovulating and had nothing to worry about. I did a lot of googling around the same time and some sources seemed to suggest lighter periods can be normal for some women as we get older. I had never been pregnant or been on the pill.
At that stage I wasn't tracking my cycle much- despite having light periods I still got fairly painful period pains giving me a heads up when I was about to come on, so never really bothered to count days. I assumed my cycles were still around 28/9 days as they had been since I was a teenager. A year or two later, started ttc and noticed they were actually more like 24 days. I didn't think anything of this. After many many months without a positive pregnancy test, I ended up having a consultation with a fertility doctor with decades of experience at a private clinic, who told me that anecdotally, they tend to find that women with shorter cycles do have lower ovarian reserves. He hypothesised that because fewer eggs are maturing each month, it takes less time for the lead egg to become fully mature and be released. I used OPKs to track ovulation and it was the first half of my cycle, before ovulation, that was significantly shorter, rather than the second half, so this checked out for me.
It can all sound a bit alarming, but ovarian reserve obviously does decrease as we get older, and as long as you are still ovulating the odd 'good quality' egg, which you normally would at 35, there shouldn't be any reason why you can't get pregnant again eventually. Egg quality is something that isn't talked about as much as quantity, or ovarian reserve, but it does decline as we age also. If in your mid-30s only 25% of your eggs are genetically viable, on average you would only be able to conceive a healthy pregnancy one month in four, so four months of trying really isn't long.
Having said that, I would recommend you follow NHS guidelines and refer yourself to the doctor if you're not pregnant after 6 months trying. In my case, although I was ovulating and had a fairly normal ovarian reserve for my age (of course significantly depleted from my younger days), it turned out one of my Fallopian tubes was blocked and the other may well be damaged and non-functioning too. This was not in any way linked to my short, light cycles and dwindling reserve, but it meant I had to have ivf to conceive. The success rates are much higher the younger you are and drop significantly even between mid and late 30s. If you have a lower ovarian reserve, it will usually take several attempts. I was 35 when we first started ttc with a very relaxed approach and 38 when my third ivf cycle finally gave us a positive pregnancy test. There were some really stressful months when I really thought it would never work as I'd left it too late and really wished I'd had investigations done sooner. Everyone's situation is unique and it's probably very unlikely you have any issues at all -fingers crossed you'll conceive soon enough. I certainly don't want to scare you by talking about me IVF when there's no reason to think you'll need it. However, knowing what I know now if anyone is asking for advice I always share my experience and recommend starting investigations sooner rather than later