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How old is your dog, because if he is middle-aged or older, the symptoms you described sound a lot like what happens to dogs who have a type of tumour called hemangiosarcoma. These are tumours that often have no symptoms until they rupture, and the dog loses blood rapidly (internally). The sudden pale gums and the soft lump that came up on his side around the same time would suggest something like this. The only way you will find out, however, is to have a necropsy done as someone already suggested.
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