It's normal. It happens because of a variety of reasons, none of which make you loopy.
Sometimes it's because it plays to a very common and normal fear that the relationship breakdown is somehow all your fault. Even when you've been cheated on/abused/whatever, you sometimes feel "if I'd tried harder/been prettier/been easier to live with/etc" your X would not have behaved so badly. The X then finding apparent happiness with someone else seems to reinforce that you were the problem, not them.
Sometimes it's because it seems so unjust. If you've been shit on from a great height and you're still reeling, it seems incredibly unfair that the person responsible for that pain gets to have a second shot at happiness. Injustice stings.
Other times it can be because of fear and guilt. If you're the one that left, for example, if you haven't moved on but he apparently has, it makes you question your decision to leave and therefore everything building up to it.
It's all normal and all part of the grieving process. Those who skip it without going through an accompanying period of self-examination and growth are less likely to have a subsequent happy relationship BTW so you may find some comfort in that.
Hope you feel better soon.