The hard truth is that 30% of pregnancies miscarry in the first 12 weeks. This is simply because the baby forms completely in the first three months, and it's quite difficult to assemble a whole baby with all the bits in the right places, first time, every time - some pregnancies are not viable because the body has forgotten to grow some vital part, or something has grown too fast or too slowly, or there is some serious birth defect that would prevent the foetus developing into a baby.
However, it is almost always nothing that the mother has done. The foetus is very well protected by your body and the uterus (which is a hard muscle and resists most knocks and blows). Also bear in mind that most women don't know they're pregnant for much of the first 12 weeks and many carry on smoking, drinking and taking things they shouldn't, yet still produce healthy babies months later. Falling over can rupture the uterus and later the placenta, which would cause bleeding, but as I said, the uterus is very tough and not easily ruptured.
Much later in the third trimester falling over can trigger premature labour. However, women who miscarry have usually done nothing to trigger it - it just happens sometimes.
Once you're past the first 12 weeks, you can begin to relax a bit - the risks go down a lot once you're into the second trimester, but for now just try to enjoy the ride and don't worry too much. One saving grace is that even if you miscarry, you are super-fertile just afterwards so you can try again straight away.
Resting at home is not going to reduce the risk of miscarriage - it's nearly always due to very serious birth defects when the baby is forming, and so I would recommend carrying on with your life until you feel that you want to stop. The first trimester is hard work, but once your'e past that 12 weeks you'll feel like your normal self again.