11.a3 was fine.
In the long run you want to play c4.
Notice how the bishop on e3 blocks your queen and rook on the e file. That's partly why white feels uncomfortable. It was better to develop it to g5 or leave it on c1.
I know beginners are told to develop pieces to squares where they influence the center, but playing both Bd3 and Be3 often runs into this problem (you're congesting the center files which annoys your heavy pieces: queen and rooks).
[Date "04/19/2020 12:42PM"]
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]
[White "Alejansro21 (600)"]
[Black "The_Calvatron (690)"]
[Result "Game drawn by stalemate"]
[Termination "Game drawn by stalemate"]
1.d4 {15:09} d5 {15:08} 2.Nc3 {15:17} Nf6 {15:03} 3.Nf3 {15:14} e6 {15:03} 4.e3 {15:03} c5 {14:48} 5.Bd3 {14:27} cxd4 {14:34} 6.exd4 {14:29} Bd6 {14:32} 7.O-O {13:50} O-O {14:38} 8.Be3 {13:57} a6 {14:27} 9.Re1 {13:21} Nc6 {14:10} 10.Qe2 {12:31} h6 {13:36} 11.a3 {12:12} Bd7 {12:36} 12.b4 {10:55} Ng4 {12:07} 13.b5 {10:26} Ne7 {10:34} 14.Na4 {8:56} axb5 {8:49} 15.Nb6 {8:36} Ra6 {8:33} 16.Nxd7 {8:27} Qxd7 {8:42} 17.Rab1 {8:20} Nxe3 {8:28} 18.Qxe3 {7:47} Nf5 {7:41} 19.Bxf5 {7:32} exf5 {7:44} 20.Ne5 {6:52} Qc6 {7:00} 21.Nxc6 {6:29} bxc6 {7:07} 22.Qf3 {6:26} g6 {7:06} 23.Qh3 {6:26} h5 {7:14} 24.Ra1 {5:47} Raa8 {6:54} 25.Rec1 {5:39} c5 {6:39} 26.Qb3 {5:32} c4 {6:31} 27.Qxb5 {5:36} Bxa3 {6:32} 28.Rcb1 {5:22} Rab8 {5:44} 29.Qa4 {5:16} Bd6 {5:13} 30.Rxb8 {5:08} Rxb8 {5:07} 31.Qc6 {5:05} Rd8 {4:02} 32.Ra8 {5:02} Rxa8 {3:32} 33.Qxa8+ {5:06} Kg7 {3:13} 34.Qd8 {5:10} Bf8 {3:11} 35.Qxd5 {5:13} Be7 {3:10} 36.Qe5+ {5:21} Bf6 {3:18} 37.Qc5 {5:23} c3 {3:22} 38.Qxc3 {5:30} h4 {3:14} 39.Qc6 {5:36} Bxd4 {3:17} 40.c3 {5:44} Bf6 {3:11} 41.Qf3 {5:45} Be5 {3:00} 42.Qh3 {5:46} Bf6 {3:06} 43.c4 {5:55} Be7 {3:03} 44.Qxh4 {6:03} Bxh4 {3:10} 45.c5 {6:12} Bd8 {3:14} 46.c6 {6:21} Kf8 {3:18} 47.g3 {6:29} Ke8 {3:26} 48.f3 {6:29} Bc7 {3:23} 49.g4 {6:33} fxg4 {3:16} 50.fxg4 {6:43} Ke7 {3:24} 51.h4 {6:50} Ke6 {3:30} 52.Kg2 {6:53} Kf6 {3:20} 53.Kh3 {6:45} g5 {3:24} 54.hxg5+ {6:31} Kxg5 {3:32} 55.Kg2 {6:22} Kxg4 {3:39} 56.Kf2 {6:31} Kf5 {3:40} 57.Ke3 {6:41} Ke6 {3:49} 58.Ke4 {6:40} Kd6 {3:58} 59.Kf5 {6:48} Bd8 {3:58} 60.Ke4 {6:17} Kxc6 {4:06} 61.Ke5 {6:26} f6+ {4:13} 62.Kf5 {6:28} Kd6 {4:20} 63.Kf4 {6:32} Ke6 {4:25} 64.Kg4 {6:36} f5+ {4:33} 65.Kf4 {6:45} Bc7+ {4:41} 66.Kf3 {6:52} Ke5 {4:45} 67.Ke3 {6:59} f4+ {4:53} 68.Kf3 {7:08} Bb6 {4:57} 69.Kg4 {7:17} Ke4 {5:03} 70.Kh3 {7:13} f3 {5:11} 71.Kh2 {7:22} f2 {5:13} 72.Kg2 {7:26} Ke3 {5:15} 73.Kf1 {7:30} Kd2 {5:21} 74.Kg2 {7:33} Ke2 {5:30} 75.Kg3 {7:32} f1=Q {5:36} 76.Kg4 {7:41} Qg1+ {5:24} 77.Kf5 {7:50} Bd4 {5:25} 78.Ke6 {7:54} Qe3+ {5:29} 79.Kd5 {7:57} Kd3 {5:33} 80.Kc6 {8:03} Kc4 {5:41} 81.Kd7 {8:07} Kc5 {5:36} 82.Kc7 {7:45} Be5+ {5:34} 83.Kb7 {7:50} Qd3 {5:23} 84.Kc8 {7:41} Kb6 {5:20} {Game drawn by stalemate}