Why has my brain felt so funny lately?

For the past week-ish, my brain has felt really weird and not normal. I pride myself on having a very good memory and being a rather quick thinker. Lately, I feel like my brain has been in a fog. I have always been forgetful about short-term events, like if somebody says "remember to do this later", I'll often forget so that's not a big deal. But I keep thinking of things I have to tell people, and then as soon as I see them I forget what it was. Also, I keep having the feeling of having a word "on the tip of my tongue", like I can think of a word I wanna use but I just can't come up with it. I haven't been getting much sleep lately, it's been very hot and humid in my area, and I really haven't been stimulating/challenging my brain much this summer...do you think that these are factors, or is it something more serious??