Could The Delayed Periods Be Due To Stress?

ok a tough question but my girlfriend had a affair and had unsafe sex about 3 weeks ago she usually has her period every 28 days she is usually never late never early. a couple of days after her affair i got hospitalized with a life or death situation so i know she was pretty stressed about the for about a week till i got out of the hospital now her period this month was 4 days late she just started today what are the chances she could be pregnant or if it was just stress that delayed her period she just turned 21 years old thank you