This is an issue which Brazilian students often have problems with. The words 'just' and 'only' are sometimes synonyms in English, but not always. 'Just' as an adverb has three meanings: 1. Very recently - 'I've just heard the news'. 2. Exactly - 'That's just what you need'. 3. Only - 'He just speaks English.' The two words are interchangeable only for the 3rd definition The two words are interchangeable just for the 3rd definition. In the sentence you have given, 'just' has the meaning of 'very recently' (definition 1) so you can't replace it with 'only' in this situation. In fact, you can say 'I only just logged in', but here the word only is simply intensifying the word 'just'. You can't use 'only' on its own in that sentence. I hope that helps.

'I just logged in' would be correct.