Should I be concerned?

In December 2017 I was diagnosed with breast cancer. In the January of 2018 I had a mastectomy left side with immediate reconstruction. I had turned fifty in the June prior to the diagnosis and so It was my first mammogram. Previous to this I had seen my GP on quite a few occasions concerned as to why I was so tired all the time. He said your fifty, your getting older, it happens. I was having problems with the peri menopause and was desperate to stop or ease the hot flushes and mood swings. I was quite low and felt so miserable a lot of the time. I had no pain or discomfort in my breast and no changes you could see. I told him there was a history of breast cancer on both sides of our family. But he said being both grandmothers it wouldn't count. So eventually he gave me HRT. I felt like a new woman. It significantly reduced the hot flushes and I felt like a well balanced happy person again. Fast forward to the mammogram, I'm told Breast cancer left side. I was shocked but had a complete work up and had the mastectomy. I feel I'm lucky as it wasn't recommended that I have chemo. Radiotherapy was considered, but the tumour was so large and near the heart it was to risky. After recovering I can honestly say it was the best thing I had done. I was mentally feeling better, physically feeling good and no more hot flushes or mood swings or feeling very depressed. Obviously I cannot any longer take HRT but not having the symptoms in the first place I haven't needed to. This last three to four weeks however has drastically changed. I don't have mood swings, I don't necessarily feel low, but my tiredness has come back with a vengeance. I feel so so weak and lethargic. I'm really struggling now to even just do the vacuuming. I started getting what feels like hot flushes but now they are every 15 mins or so and they are raging. I have a constant headache and I feel like my legs are going to give out. It all sounds quite pathetic but it couldn't be more real. I feel so poorly. I'm now beggining to wonder if I should get this checked out or if my cancer has returned. I never had mets, I was given the all clear on that score. And it's only been a year and a half since it all happened. I have spent the weekend literally sleeping. And whilst I am feeling a bit better today, I'm not right and struggling physically. I also feel quite disconnected. I've got the fan on almost constantly to try and regulate my temperature. Could I be going through another phase of the menopause or something else? I'm surprised I've stayed awake long enough to write this I'm so tired. And if that's what the menopause amounts to it's horrendous. Yes I'm concerned. I have no changes that I can see in my reconstruction and nothing on the other side. But this tiredness, lethargy and complete and utter exhaustion is overwhelming.