Alshammar won her first international medal, a bronze in the 50 m freestyle, at the 1997 European Championships. She developed this stroke into her speciality and won the silver at the 1999 European Championships. She broke through to the top ranks at the 2000 Summer Olympics in Sydney where she won silver medals in the 50 m and 100 m freestyle, both times vanquished by Inge de Bruijn, and a bronze medal with the Swedish relay team.