Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Distraction; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Unstabilized Approach
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Human Factors
On the RNAV (RNP) Runway 19 arrival; after completing the checklist; fully configured at GREYZ; somewhere between I believe JUBOL and WIRSO we received an EGPWS 'Caution Obstacle' alert. Simultaneously the tower called and said we were low on our altitude. The First Officer tried to respond to the tower but mistakenly pushed the autopilot disconnect button creating a confusion for me as the flying pilot as to what just happened. At that point; I started to hand fly the aircraft back onto the proper course and altitude for the remainder of the approach. As I was climbing; the aircraft back to a higher altitude we got slow and had to add power. With the nose up attitude; there was a momentary disorientation as to where the runway was located. Found the approach lights and VASI; corrected for being high and made a normal landing. Neither the First Officer nor I can understand how the aircraft got into this situation.  We had pre briefed the approach and checked all the points. Everything seemed normal from GREYZ and SETOC points on the approach. This is obviously not acceptable and I would greatly appreciate it if someone would look into this so I can find out if it was human error or FMC error.
B737 Captain reported simultaneously receiving an EGPWS 'Caution Obstacle' alert and an ATC low altitude warning while on approach into DCA.