I do not suffer from PVC, but what I have read suggests to me your symptoms are "typical"... still you should discuss any changes with your doctor, cardiologist in this case I think. The fact that you can relate the episodes with aerobic physical activity such as tennis but not with the "tread mill" is a mystery to me too.
For my part I would stop physical activity, maybe do a cool-down first if possible, I would not ignore the thumps if more than a couple.
If the frequency becomes "too" frequent you should consider an ablation, at least discuss it with your doctor. I have had discussions on an ablation with my doctors to stop my atrial fibrillation, but they (two of them) recommend against it. This is because there are some risks associated with ablation, more for AFib than for PVC, so they don't see the benefit verses risk worth it. I think the same analysis applies (with a lower number for the risk part) consideration of a PVC... also you young age is a factor in favor of ablation, I am a "senior". In the US I think that can also be a factor, while I am on the traditional Medicare insurance coverage, with a private secondary plan, I expect to get the Medicare (low) approved rates for medical treatment, so the doctors don't make get paid only a fraction of their full charge. I am confident I am not at more risk for not having an ablation done, but I am suggesting that insurance coverage (hope you have it) can play a role.