Is it normal to forget your native language?

So I’ve been learning a few different languages recently, and I therefore hardly ever think in English. At college, I’ll be daydreaming and somebody will talk to me in English and I’ll genuinely not understand what they’re saying because I’m so concentrated on something else. I keep forgetting simple grammar and what something is called. My reading has slowed dramatically, because reading a Russian ш is pronounced sh, so I struggle with W’s. It’s the same with н(n), р(r), в(v), ц(ts), у(u), е(ye), г(g), m(t), n(p), g(d).

It’s just so annoying!