Was the nausea worse in the morning, gradually getting better by the evening when adrenaline calms down? That can give you a clue if it's hormone related, as a hit of artificial hormones can cause nausea (even prog as morning after pill commonly does this) but if it's infection it's likely to be 24/7 and you'd have fever and pain.
If you Google, many report nausea with Mirena at first. I also had ABs when didn't know symptoms were all hormonal as surges caused sinusitis dizziness etc. and friends were swearing to me the nausea was from the Amoxy, but like you, I never had probs with them as they speed up transit and I needed that so always felt great on Amoxy!
Don't go by your last experience with Mirena as you're probably in the mid surging phase of peri so you can't compare one month to the next whether taking hormones or not. Maybe up your Estrogel whilst your body gets used to the hit of prog as that might oppose it enough.
