The incident occurred in the second half of Portsmouth's 2-0 win at Fratton Park when Bielik went to retrieve the ball from a stand packed with Pompey supporters.
Video footage showed Bielik becoming involved in an altercation with a teenage boy, who caught the ball and then appeared to be pushed in the throat by the Cardiff player after he had thrown it back towards the pitch.
Cardiff boss Brian Barry-Murphy claimed Bielik was slapped in the face by a fan, while Portsmouth manager John Mousinho apologised to the Welsh club for the incident.
In Sunday's Championship action, Wolves were held to a 2-2 draw by Birmingham in a frenetic local derby at St Andrew's.
Hoping for an immediate return to the Premier League after last season's relegation, Wolves twice took the lead through Fer Lopez and Ladislav Krejci.
But Jay Stansfield bagged Birmingham's first equaliser and a Jackson Tchatchoua own goal hauled the hosts level again by half-time.
Wolves striker Adam Armstrong wasted a golden opportunity in the second half, to the relief of Birmingham owner Tom Wagner, who was watching from the directors' box for the first time since he had a stroke in February.