Sounds like you used too much onion and cilantro probably. Though, I usually find that cilantro gives it a soapy flavor, if its overused.
Try this mix together a bit of garlic, pepper and rotel tomatoes, preferably the kind that has green chillies in it. Then add it to the other salsa and puree it all together. It should fix the flavor. You can add a bit more lime juice, or finely grated rind to it also.
Good luck!