I always get erected when see a woman nurse and even by looking a syringe. whether this's normal issue or i've mental problem? is it bad at all?

You're fine, man: You're a 25 year old man. Every guy in the world has some unexplainable turn-ons that make no sense. Yours is tame, non-violent, and kind of sweet. Life's difficult enough without troubling yourself over your private thoughts. Embrace who you are. Perhaps there's a special woman in your life that can role-play your fantasy. Resolve in any case to love wisely and responsibly.