It's certainly more common to have pain and swelling with an ACL injury, but it's not unheard of not to have any. I'm afraid the instability is a much stronger indication that something has happened to the ACL. Fingers crossed, though - it's also possible to be showing all the signs of an ACL tear, and then find out via MRI or scope that it's actually intact. I very much hope that this is the case for you.