first, as an answer: yes! it is a common occurrence for bilingual speakers. Wanting to say a word and your brain providing only the english (or whatever other language) for it, or using idiomatic expressions that don't make sense in the other. It doesn't mean you're going to lose your ability in russian. If you have a test, keep studying for it. That something like this is happening is a good signal in a sense. But when you keep progressing, read more in russian and practice so you keep both languages in good condition