If a person used to talk to you, but not anymore (which is avoiding you), and if I have something to get off my chest, should I do so?

You can absolutely talk to this person about why they've been distant lately and how you feel about it. However, yelling at them or being accusatory might just make the situation worse. It's better to confront this person respectfully than angrily react to their avoidance, as hurtful as it can be. If you want to try and mend the relationship, you could say, "It feels like you've been ignoring me lately. Did I do something to upset you?" Even if you just want to express how you feel and put the relationship behind you, you could say, "I feel like you've been avoiding me lately. I'm not sure what happened, but I feel like both of us might need some space for a while."