It would be very unusual for physical activity to cause bleeding. However it is very common to have some slight bleeding in early pregnancy. Your hcg levels are good, and as others have said it is difficult to see much on a US before 6 weeks. When you are doing physical working outdoors it is very important to ensure that you have plenty of fluids. I would suggest that you should have at least 3 litres a day when working. Another tip to remember do not wait until you are thirsty to have a drink. When you start to feel thirsty you are already starting to dehydrate