Slow Hot Water Since Turning on Heat - Tankless

Having an issue here and no amount of googling has pointed me in the right direction We have a 2 year old oil burner with tankless on demand hot water heating. All of last year we had zero issue with getting hot water extremely quickly whenever turning on the shower, or sink, etc. No more than 10-15 seconds and it was boiling hot. Even during the BRUTAL sub zero polar vortex temps we had last winter, the shower always heated up very quickly. Temps have been jumping all over the place here in NY over the last 2-3 weeks, and finally about a week or so ago it got fairly cold so we needed to turn on the heat. Turned the aquastat up to 185/165 (high/low) which is my usual early winter settings, up from 165/145 for summer. We have a 2 zone heating system, and we turned both zones on for a couple of days, and then turned 1 off (second floor) when our guests left since we primarily live on the first floor, and the second floor is more storage and guest space. Since then, we've had just the first floor zone on, set to around 65 degrees. The house seems to be keeping steady heat, and the radiators are warm to the touch as normal, however since turning on the heat a couple of weeks ago, we now have a shower that takes quite a while to heat up, and even then when it does get hot, it requires you to turn it to almost full hot, where as it used to be comfortable at 75%. Also the first day after we turned the heat on, when turning the shower on i got a definite quick pipe vibration, that happened again a few days later, however hasnt happened since. I occasionally now hear an occasional pipe bang (water hammer sound) every so often when the heat kicks on as well. A few days into the problem, i tried turning the upstairs thermostat back on set to around 63, thinking the water might be circulating and cold water from upstairs might be making it work harder to reheat, but it hasnt changed anything and both zones have now been on for over 4 days. Thoughts as to what could be causing this sudden issue with very slow shower water heating? All last winter it was NEVER a problem. We've owned the house now for over a year and a half and this is the first time this issue has ever arisen