Sudden continuous HR spikes

hello! I’ve (M25) been training quite consistently for my first marathon and generally have a low HR: ~53 resting HR and never more than 170 even when doing intervals. For the last couple of days, my HR has been spiking. I went for a run this morning and it averaged 175 at 5:00/km (my tempo pace is 4:10!). Resting HR is also in the 80s. I’ve also had intermittent chest pain on the left side.

Any idea of what that could be? I am not ill and have had good sleep / nutritional intake, but am getting super worried one month out of my first marathon.

Thanks all!