Could be belts, make sure they are tight. Could be stepper currents/over heating causing it to skip steps. Did anything touch the bed while printing? could of misaligned it. Also check the gears on the steppers, make sure the grub screws are tight.
Good Luck! also you could contact Wanhao USA they are pretty good with customer service
Check the obvious first, turn off power, slide table in all axis, notice rumbling, change bearings out for proper ones or get someone to print you plastic ones. the i3 is notorious for crap bearings, don’t worry its easy to get new ones, or have you got crap falling off the bed onto guide bars,motor. good maintainense is essential to good prints.
Similar thing happened on my i3v2 and it was time to re-square my printer. The Z frame had become misaligned with everything else and found my entire base was out of square too. Drove me nuts for quite a while. Seems too soon to have all that happen with your Plus but I would take a good square and level (and digital caliper) and find out what’s out of alignment with everything else.
See, dosent it feel good to have found it yourself. while you’ve got it off. put an extra brkt on idler wheel, surprising how much this can flex. May stop future probs.