And what was your flight plan and/or selected altitude for that portion of the flight?
For example, if you look in your Flight Plan page, and click on the VNAV button on that page, you will see Climb profiles … in there are certain restrictions that below a certain altitude, and within X miles of the airport, it will stay below a certain speed. Eg, below 10,000 feet, below 250 kias.
Also if you use TOGA to take off, after you reach +400 feet AGL, you should change to FLCH (or VS) to your selected altitude… If you leave it in TOGA (which you shouldn’t) the plane will often overspeed during the initial take off if you are not careful, then as you change modes, the VNAV profile restrictions may be telling it to slow down … and you feel the loss of power. Try to watch what the scoreboard and bugged speed targets are at that point.
Also … check your flaps? Speed is automatically limited / reduced by the amount of flaps and if you have your gear down.