Why did my thai curry have a bitter aftertaste?
Disclaimer: I have no experience with low FODMAP diets. But it seems quite a challenge to make a Thai curry without garlic or shallots in any case! 

In Thai cooking, you typically want to have very strong flavours, and a balance of the basic flavours: bitter, sweet, salt, sour, and umami. So perhaps your problem isn't so much having a strong bitter taste (which isn't appreciated as much in western cuisine as it is in Thai cuisine), as a relative lack of the other basic tastes?

I would suggest that you may be missing out especially on salt and umami, because you haven't used any fermented fish sauce (which is typically very salty) for seasoning, nor fermented shrimp paste ("gapi", which is very umami) in the curry paste. One teaspoon of salt is not very much; in commercial curry pastes you may find as much as 25% salt.

Other things that come to mind (which may or may not be relevant to your issue):


Don't use lime juice in the curry paste; but do use a bit of lime zest. It's fine to use lime juice for seasoning while cooking the curry, though. I typically use a bit more than one teaspoon, perhaps two or three. But it all depends on the balance of the dish: as a rule of thumb, add (palm) sugar for sweetness, fish sauce for saltiness, and lime juice for sourness, until you can taste all basic tastes, without any one overwhelming the others.
Don't include the coriander leaves and top part of the stems in the curry paste, only the roots and lower part (about an inch) of the stems. You can use the leaves to garnish the dish.
Don't use spring onions in the paste, as they make a poor substitute for shallots; they're also better used for garnishing the dish. 
Use galangal instead of ginger, if you can find it (if I can't get fresh galangal, I won't even bother trying to make my own curry paste).


Hope this helps!