First time not getting hard after 3 months of good sex

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So I’m having a stressful week and it affects my libido. For the first 3 months of our relationship I always wanted to have sex and was getting aroused really quickly. Since last week, I don’t seem to get aroused very much and I’m a bit scared that my girlfriend notice it and start asking questions about it. Since I’m a bit stressed about it, my erection aren’t as hard and I’m more into my head before sex. I’m pretty sure it’s just a phase because I’m stress about work stuff and I don’t want to start a performance anxiety process with my girl so what should I do ?