I agree that it would most likely have to do with your render settings. Or that you have something else running on the PC, taking up some of your resources.
When this happens to me, I will close Apo and reopen it, to find that the memory has increased. Most times, decreasing the oversample setting can remedy the lack of physical memory. I use a setting of 1 or 2.
Your image seems quite dark, and I find it difficult to see the details of the flame. If you increased the image prior to uploading here, this may be the reason.
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Every once in a while I have that problem & I usually save the parameters & either render it the smaller size or come back later & try again.