Woman sighs on the bus as I move away from her crying baby

I just got on a public bus to work in Galway and was very tired. I had just sat down, the bus had not pulled off from my stop yet and I had took my charger out of my bag was about to plug it in. The bus was completely empty and I sitting at the very back. Directly after me, up the bus stairs comes a woman and her 2-4 year old child. Her child came jogging down the empty aisle, chattering and stomping as he went. Out of all the empty seats, he chose the ones behind me. Trying not to seem rude, I pretended to fiddle with my phone charger at the socket as if it was not working. I picked up my bag and began to move to another seat, it was silent and I heard the woman let out a loud, exaggerated sigh, followed by a sharp “really?!”, as if my quiet escape had personally offended her. The whole thing was so awkward I just stared out the window and pretended not to hear as the bus took off. How rude on her behalf I’m thinking upon reflection.