My motherinlaw had heart palpitations then a surgery to be monitored and told to stay off her feet then developed a blood clot what could cause this?

Stasis: That is a packed question. Assuming you mean the clot, stasis of blood caused by prolonged periods off your feet is a risk factor. Depending on the other issues, bed rest may well have been indicated- it just goes to show that there is no completely benign intervention, even staying off your feet has risks. Clots that are identified promptly can be treated to prevent further harm.

Happens with bedrest: Blood clots or deep venous thrombosis can happen at any age but the elderly are at greater risk. I will refer this to a cardiologist.