This message means that all high ports (>1023) are used up for outbound NATted connections. This is very rare and signifies that your FGT is processing way too many sessions. Check the number of open sessions in the GUI or CLI (get sys perf stat), "average sessions" and "average sessions setup rate".
Two main reasons may apply: either you've hit a firmware bug or some host(s) on your internal network start sessions wildly, possibly malware induced. High (outbound) bandwidth consumptions hints into the malware direction, too.
Remedy: reboot your FGT. Watch the session count and buildup rate. Depending on the firmware version used, check FortiView to determine where the majority of sessions either originate or are destined to. Then follow the trails.
You may find more hints and help on the Fortinet forum (forum.fortinet.com).