Weird Sex Problem...what Could Explain This?

I'm 20/virgin/have been masturbating once a day. In the past 6 months i've been having difficulty getting or maintaining an erection. I also tend to have low libido in the days that I can't get an erection. What is weird is that, in the past 2 years, which means this existed well before I started having erection problems, there's been a sexual cycle going on in me. 2-5 days in a row out of maybe 2-4 weeks, I have a stronger libido and indescribably better erections. I call those days my "horny days." of course I thought it was normal because I was still able to get hard in other days. But since 6 months ago when I started to experience difficulties, those days have become the only times I could get really hard and last long to masturbate, though I still ejaculate with a semi-hard penis in other days. I'm just totally off. I have ruled out factors such as stress, health, the porn I watch, how turned on I am etc. Because they are basically constant. I mean in horny days pulling off my pants and touching my penis or just thinking about porn is enough to make me rock hard. And this cycle is just too regular: it comes in a certain interval and I can't make it last or change the pattern. It doesn't matter if I masturbate 4 times a day or don't do it at all in those days. Also the horny days are continuous i.E. It wouldn't be like, one day i'm horny, the next i'm not, and then the next day i'm horny again. It just never changed its pattern. Not even once.

I've gotten my testosterone and thyroid checked and everything is normal. I've been taking zinc and multi-vitamins with no results. The only thing I think could explain it is hormonal imbalance or disorder, like I have high hormone level in horny days so I get horny and erections last. It's a biological cycle sorta like a woman would experience (weird I know). But the doctor said that wasn't possible...That hormones can't fluctuate like that. I don't know. Nobody could explain it. I myself know it's physical. I mean hey this cycle has been there for two years and even before I became impotent!