Her first international competition was the 2004 IAAF World Junior Championships, where she finish fourth in her heats. Kowalska won the steeplechase gold at the 2005 European Athletics U23 Championships. In 2006, she won a bronze in the European Cup and at the 2006 European Athletics Championships she recorded a personal best of 9:42.50 in the qualifiers before going on to finish ninth in the final. She represented Europe in the 5000 metres at the 2006 IAAF World Cup in Athens but finished in last place. She ran at the 2006 European Cross Country Championships in the under-23 race and although she finished ninth, she headed Poland to the team silver medal.