No not nuts just jealous and rightly so as long as you keep it in check. If you weren't jealous something would be wrong with you because there is a history there and like they say history repeats itself.
As long as you trust him to not cheat and the situation doesn't make you too uncomfortable I wouldn't worry. Talking more than twice a week is too much though and if he does this with her I would give him an ultimatum because even twice a week is pushing it.
If this girl IS trying to get with your man though I would consider asking him to not talk on the phone so much to her or not at all because if he has feelings for you he should understand you don't trust her and he is putting you in an awkward situation.
There is really no reason though for you to have to worry about another girl and if he doesn't understand this and do something about it then hes not worth it hun. Relationships have enough problems without someone trying to butt in them all the time. In order for you two to move on he has to give up the past if it bothers you.
One day the Moon said to me, "If he makes you cry, why dont you leave him?" I looked up at the Moon and said, "Moon would you ever leave your sky?"